Sensor element and gas sensor

ABSTRACT

A sensor element includes an element body having an elongate rectangular parallelepiped shape and including solid electrolyte layers with oxygen ion conductivity, an outer pump electrode disposed on a first surface of the element body, and a protective layer covering at least a part of the first surface of the element body and including one or more exposed spaces (an upper space) to which the first surface is exposed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sensor element and a gas sensor.

2. Description of the Related Art

There is so far known a gas sensor including a sensor element thatdetects the concentration of a specific gas, e.g., NOx, in measurementobject gas, such as automobile exhaust gas. It is also known to form aprotective layer on a surface of the sensor element in that type of gassensor. For example, Patent Literatures (PTLs) 1 and 2 disclose that theprotective layer is formed on the surface of the sensor element byprinting or plasma spraying. According to the disclosures of PTLs 1 and2, cracking, etc. of the sensor element attributable to adherence ofmoisture in the measurement object gas, for example, can be suppressedby forming the protective layer.

CITATION LIST Patent Literature

[PTL 1] JP 2011-214853 A

[PTL 2] JP 3766572 B

SUMMARY OF THE INVENTION

A temperature of the above-mentioned sensor element in the gas sensor ishigh (e.g., about 800° C.) in an ordinary drive mode. Thus, it isdemanded to further suppress cracking of the sensor element, which mayoccur due to quick cooling with adherence of moisture.

The present invention has been made with intent to solve theabove-described problem, and a main object of the present invention isto improve moisture resistance of an element body of a sensor element.

To achieve the above main object, the present invention is constitutedas follows.

A sensor element according to the present invention includes an elementbody having an elongate rectangular parallelepiped shape and includingsolid electrolyte layers with oxygen ion conductivity,

an outer electrode disposed on a first surface that is one of surfacesof the element body, and

a protective layer covering at least a part of the first surface of theelement body and including one or more exposed spaces to which the firstsurface is exposed.

In the sensor element described above, the protective layer covers atleast a part of the first surface among the surfaces of the element bodyon which the outer electrode is disposed. Furthermore, the protectivelayer includes one or more exposed spaces to which the first surface isexposed. With those features, since heat conduction in a direction ofthickness of the protective layer can be blocked off by the exposedspaces, cooling of the element body is suppressed when moisture adheresto the surface of the protective layer. Hence the moisture resistance ofthe element body is improved.

In the sensor element according to the present invention, at least oneof the exposed spaces may be positioned in an overlapping relation tothe middle of a region of the first surface, the region being coveredwith the protective layer, when viewed from a direction perpendicular tothe first surface. Here, temperature in the middle of the region coveredwith the protective layer is comparatively more apt to become high, andtemperature in a part of the relevant region farther away from themiddle (e.g., a part nearer to an end of the first surface) iscomparatively less apt to become high. Therefore, the moistureresistance of the element body is further improved by heat-insulatingthe middle of a region of the first surface, the region being coveredwith the protective layer, by the exposed space. It is to be noted thatthe “middle of a region of the first surface, the region being coveredwith the protective layer” may be the middle of the region covered withthe protective layer in a short-length direction of the first surface,or the middle of the relevant region in a longitudinal direction of thefirst surface, or the middle of the relevant region in each of theshort-length direction and the longitudinal direction of the firstsurface (i.e., the center of the region covered with the protectivelayer).

In the sensor element according to the present invention, at least oneof the exposed spaces may be positioned such that at least a part of theone exposed space overlaps at least a part of the outer electrode whenviewed from the direction perpendicular to the first surface.Temperature of the outer electrode is comparatively more apt to becomehigh. Therefore, the moisture resistance of the element body is furtherimproved by heat-insulating at least the part of the outer electrode bythe exposed space.

In the sensor element according to the present invention, at least oneof the exposed spaces may be provided with an opening in communicationwith the outside of the protective layer. With that feature, since heatin the exposed space can be released through the opening, overheating ofthe element body is suppressed. As a result, an abrupt temperature dropof the element body can be suppressed when moisture adheres to thesurface of the protective layer, and the moisture resistance of theelement body is improved.

In the sensor element according to the present invention, the protectivelayer may include the plurality of exposed spaces, and a plurality ofnon-exposed spaces to which the first surface is not exposed, and whichare present at positions deviated from at least one of the exposedspaces in the direction perpendicular to the first surface. When thoseplural spaces are present in the protective layer, strength of theprotective layer is less apt to reduce in comparison with the case wherethe protective layer includes one space having the same total volume asthat of the plural spaces. As a result, reduction in the strength of theprotective layer attributable to the presence of the spaces can besuppressed while the moisture resistance is improved with the provisionof the spaces in the protective layer.

In the sensor element according to the present invention, the protectivelayer may include, for at least one of the exposed spaces, one or morepillar portions that hold the exposed space in the directionperpendicular to the first surface. With that feature, since the pillarportions hold the exposed space, the reduction in the strength of theprotective layer can be suppressed.

In the sensor element of the type including the pillar portions,according to the present invention, the protective layer may include theplurality of pillar portions, and the pillar portions may be arrangedwith a tendency that a density of the pillar portions graduallyincreases from the middle of a region of the first surface, the regionbeing covered with the protective layer, toward a position farther awayfrom the middle when viewed from the direction perpendicular to thefirst surface. Here, temperature in the middle of the region coveredwith the protective layer is comparatively more apt to become high, andtemperature in a part of the relevant region farther away from themiddle (e.g., a part nearer to an end of the first surface) iscomparatively less apt to become high. Therefore, by arranging thepillar portions at a higher density in a zone where temperature iscomparatively less apt to become high, reduction in the heat insulationeffect of the exposed space attributable to the presence of the pillarportions can be suppressed in the region where temperature is more aptto become high, while reduction in the strength of the protective layeris suppressed by the pillar portions. It is hence possible to not onlyfurther improve the moisture resistance of the element body, but alsofurther suppress the reduction in the strength of the protective layer.The above expression “a tendency that a density of the pillar portionsgradually increases” contains a tendency that the number of the pillarportions per unit area gradually increases, and a tendency thatthicknesses of the pillar portions gradually increase.

In the sensor element of the type including the pillar portions,according to the present invention, the protective layer may include theplurality of pillar portions, and the pillar portions may be arrangedwith a tendency that a density of the pillar portions graduallyincreases toward a position farther away from the outer electrode whenviewed from the direction perpendicular to the first surface. Here,temperature of the outer electrode is comparatively more apt to becomehigh, and temperature in a portion of the first surface where the outerelectrode is not disposed is comparatively less apt to become high.Therefore, by arranging the pillar portions at a higher density in azone farther away from the outer electrode where temperature iscomparatively less apt to become high, the reduction in the heatinsulation effect of the exposed space attributable to the presence ofthe pillar portions can be suppressed in the portion including the outerelectrode where temperature is more apt to become high, while thereduction in the strength of the protective layer is suppressed by thepillar portions. It is hence possible to not only further improve themoisture resistance of the element body, but also further suppress thereduction in the strength of the protective layer.

The above expression “a tendency that a density of the pillar portionsgradually increases toward a position farther away from the outerelectrode” contains a tendency that the density of the pillar portionsis higher at a position not overlapping the outer electrode than at aposition overlapping the outer electrode when viewed from the directionperpendicular to the first surface. In such a case, the pillar portionsmay be arranged in a relation not overlapping the outer electrode whenviewed from the direction perpendicular to the first surface.

In the sensor element of the type including the pillar portions,according to the present invention, the plurality of pillar portions maybe arranged with a tendency that a density of the pillar portionsgradually increases toward a position nearer to the middle of a regionof the first surface, the region being covered with the protectivelayer, when viewed from the direction perpendicular to the firstsurface.

In the sensor element of the type including the pillar portions,according to the present invention, the protective layer may include theplurality of pillar portions, and the pillar portions may be arrangedwith a tendency that a density of the pillar portions graduallyincreases toward a position nearer to the outer electrode when viewedfrom the direction perpendicular to the first surface.

In the sensor element according to the present invention, the protectivelayer may include the plurality of exposed spaces each having alongitudinal direction aligned with the longitudinal direction of thefirst surface and disposed side by side along the short-length directionof the first surface. With the feature that the plural exposed spacesbeing elongate in the longitudinal direction of the first surface arepresent side by side along the short-length direction of the firstsurface, stress generated, due to a difference in thermal expansioncoefficient between the protective layer and the element body whenexposed to moisture, in the short-length direction of the first surfaceand applied from the protective layer to the element body can bereduced. As a result, the element body is less susceptible to crackingwhen exposed to moisture, and the moisture resistance of the elementbody is further improved. In this case, the exposed space having alongitudinal direction aligned with the longitudinal direction of thefirst surface may be a space having a rectangular parallelepiped shapein which one side along the short-length direction of the first surfaceis shorter than the other two sides.

In the sensor element according to the present invention, the protectivelayer may include the plurality of exposed spaces each having alongitudinal direction aligned with the short-length direction of thefirst surface and disposed side by side along the longitudinal directionof the first surface. With the feature that the plural exposed spacesbeing elongate in the short-length direction of the first surface arepresent side by side along the longitudinal direction of the firstsurface, stress generated, due to the difference in thermal expansioncoefficient between the protective layer and the element body whenexposed to moisture, in the longitudinal direction of the first surfaceand applied from the protective layer to the element body can bereduced. As a result, the element body is less susceptible to crackingwhen exposed to moisture, and the moisture resistance of the elementbody is further improved. In this case, the exposed space having alongitudinal direction aligned with the short-length direction of thefirst surface may be a space having a rectangular parallelepiped shapein which one side along the longitudinal direction of the first surfaceis shorter than the other two sides.

In the sensor element according to the present invention, the protectivelayer may include a plurality of first exposed spaces that are each theexposed space having a longitudinal direction aligned with thelongitudinal direction of the first surface, and that are disposed sideby side along the short-length direction of the first surface, and aplurality of second exposed spaces that are each the exposed spacehaving a longitudinal direction aligned with the short-length directionof the first surface and intersecting the first exposed spaces, and thatare disposed side by side along the longitudinal direction of the firstsurface. With that feature, stresses generated, due to the difference inthermal expansion coefficient between the protective layer and theelement body when exposed to moisture, in the short-length direction andin the longitudinal direction of the first surface and applied from theprotective layer to the element body can be both reduced. As a result,the element body is less susceptible to cracking when exposed tomoisture, and the moisture resistance of the element body is furtherimproved.

In the sensor element according to the present invention, at least oneof the exposed spaces may have a shape with a tendency that the exposedspace gradually narrows toward a position farther away from the firstsurface. With the exposed space having the above-mentioned shape, thereduction in the strength of the protective layer can be suppressed incomparison with, for example, a rectangular parallelepiped space havingan inner surface parallel to the first surface.

In the sensor element according to the present invention, at least oneof the exposed spaces may have at least two inner surfaces inclined insuch directions that the inner surfaces come closer to each other towarda position farther away from the first surface. With the exposed spacehaving the above-mentioned inner surfaces, the reduction in the strengthof the protective layer can be suppressed in comparison with, forexample, a rectangular parallelepiped space having an inner surfaceparallel to the first surface.

In the sensor element according to the present invention, at least oneof the exposed spaces may have an inner surface opposing the firstsurface and formed as a curved surface projecting outward. With theexposed space having the above-mentioned inner surface, the reduction inthe strength of the protective layer can be suppressed in comparisonwith, for example, a rectangular parallelepiped space having an innersurface parallel to the first surface.

A gas sensor according to the present invention includes theabove-described sensor element according to any one of the aspects.

The gas sensor includes the above-described sensor element of thepresent invention according to any one of the aspects. Therefore, anadvantageous effect can be obtained which is similar to that obtainedwith the above-described sensor element of the present invention, e.g.,the advantageous effect of improving the moisture resistance of theelement body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically illustrating one example ofconstitution of a sensor element 101 according to a first embodiment.

FIG. 2 is a sectional view schematically illustrating one example ofconstitution of a gas sensor 100 according to a first embodiment.

FIG. 3 is a sectional view taken along B-B in FIG. 1.

FIG. 4 is a sectional view taken along C-C in FIG. 3.

FIG. 5 is a sectional view taken along D-D in FIG. 3.

FIG. 6 is a sectional view of a sensor element 101A according to asecond embodiment.

FIG. 7 is a sectional view taken along E-E in FIG. 6.

FIG. 8 is a sectional view of a sensor element 101B according to a thirdembodiment.

FIG. 9 is a sectional view taken along F-F in FIG. 8.

FIG. 10 is a sectional view of a sensor element 101C according to afourth embodiment.

FIG. 11 is a sectional view taken along G-G in FIG. 10.

FIG. 12 is a sectional view of a sensor element 101D according to afifth embodiment.

FIG. 13 is a sectional view taken along H-H in FIG. 12.

FIG. 14 is a sectional view of a sensor element 101E according to asixth embodiment.

FIG. 15 is a sectional view taken along I-I in FIG. 14.

FIG. 16 is a sectional view of a sensor element 101F according to aseventh embodiment.

FIG. 17 is a sectional view taken along J-J in FIG. 16.

FIG. 18 is a sectional view taken along K-K in FIG. 16.

FIG. 19 is a sectional view of a sensor element 101G according to aneighth embodiment.

FIG. 20 is a sectional view taken along L-L in FIG. 19.

FIG. 21 is a sectional view of a sensor element 101H according to aninth embodiment.

FIG. 22 is a sectional view taken along M-M in FIG. 21.

FIG. 23 is a sectional view of a sensor element 101I according to atenth embodiment.

FIG. 24 is a plan view of a part of the sensor element 101I near itsfront end.

FIG. 25 is a sectional view of a sensor element 101J according to aneleventh embodiment.

FIG. 26 is a plan view of a part of the sensor element 101J near itsfront end.

FIG. 27 is a sectional view of a sensor element 101K according to atwelfth embodiment.

FIG. 28 is a plan view of a part of the sensor element 101K near itsfront end.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

Embodiments of the present invention will be described below withreference to the drawings. FIG. 1 is a perspective view schematicallyillustrating one example of constitution of a sensor element 101included in a gas sensor 100 according to a first embodiment. FIG. 2 isa sectional view schematically illustrating one example of constitutionof the gas sensor 100. A section of the sensor element 101 illustratedin FIG. 2 is taken along A-A in FIG. 1. FIG. 3 is a sectional view takenalong B-B in FIG. 1. In FIG. 3, details of the inner side in the sectionof an element body 102 are omitted. FIG. 4 is a sectional view takenalong C-C in FIG. 3. FIG. 5 is a sectional view taken along D-D in FIG.3. The sensor element 101 has an elongate rectangular parallelepipedshape. A longitudinal direction of the sensor element 101 (right-leftdirection in FIG. 2) is defined as a back-forth direction, and adirection of thickness of the sensor element 101 (up-down direction inFIG. 2) is defined as an up-down direction. Furthermore, a widthdirection of the sensor element 101 (i.e., a direction perpendicular toboth the back-forth direction and the up-down direction) is defined as aright-left direction.

The gas sensor 100 is mounted to a part of piping, e.g., an exhaust gaspipe in a vehicle, and is used to measure the concentration of specificgases, e.g., NOx and O₂, contained in exhaust gas that is measurementobject gas. In this embodiment, it is assumed that the gas sensor 100measures the concentration of NOx as one example of the concentration ofthe specific gas. The gas sensor 100 includes the sensor element 101.The sensor element 101 includes a sensor element body 102, and a porousprotective layer 84 covering the sensor element body 102. The sensorelement body 102 represents a portion of the sensor element 101 otherthan the protective layer 84.

As illustrated in FIG. 2, the sensor element 101 is an element having astructure in which six layers, i.e., a first substrate layer 1, a secondsubstrate layer 2, a third substrate layer 3, a first solid electrolytelayer 4, a spacer layer 5, and a second solid electrolyte layer 6, thoselayers being each made of a solid electrolyte with oxygen ionconductivity, such as zirconia (ZrO₂), are successively stacked in thementioned order from the lower side when viewed in the drawing. Thesolid electrolyte forming each of those six layers is a dense andair-tight substance. The sensor element 101 is manufactured, forexample, by carrying out predetermined processing, printing of a circuitpattern, etc. on ceramic green sheets corresponding to the six layers,respectively, stacking those ceramic green sheets, and then firing thestacked sheets into an integral body.

In one end portion (front end portion) of the sensor element 101 andbetween a lower surface of the second solid electrolyte layer 6 and anupper surface of the first solid electrolyte layer 4, a gas inlet 10, afirst diffusion rate-controlling portion 11, a buffer space 12, a seconddiffusion rate-controlling portion 13, a first inner cavity 20, a thirddiffusion rate-controlling portion 30, and a second inner cavity 40 aresuccessively formed adjacent to each other in the mentioned order in athoroughly communicating state.

The gas inlet 10, the buffer space 12, the first inner cavity 20, andthe second inner cavity 40 are each an inner space of the sensor element101, which is formed by boring the spacer layer 5, and which is definedat its top by the lower surface of the second solid electrolyte layer 6,at its bottom by the upper surface of the first solid electrolyte layer4, and at its sides by lateral surfaces of the spacer layer 5.

The first diffusion rate-controlling portion 11, the second diffusionrate-controlling portion 13, and the third diffusion rate-controllingportion 30 are each provided as two horizontally-elongate slits (eachhaving an opening with a longitudinal direction thereof being adirection perpendicular to the drawing). A region spanning from the gasinlet 10 to the second inner cavity 40 is also called a gas flowingportion.

At a location farther away from the one end side of the sensor elementthan the gas flowing portion, a reference gas introducing space 43 isformed between an upper surface of the third substrate layer 3 and alower surface of the spacer layer 5, and is defined at its sides bylateral surfaces of the first solid electrolyte layer 4. For example,the atmosphere is introduced to the reference gas introducing space 43as reference gas when the concentration of NOx is measured.

An atmosphere introducing layer 48 is a layer made of porous ceramic.The reference gas is introduced to the atmosphere introducing layer 48through the reference gas introducing space 43. Furthermore, theatmosphere introducing layer 48 is formed in covering relation to areference electrode 42.

The reference electrode 42 is an electrode that is formed in a statesandwiched between the upper surface of the third substrate layer 3 andthe first solid electrolyte layer 4. As described above, the atmosphereintroducing layer 48 in communication with the reference gas introducingspace 43 is disposed around the reference electrode 42. As describedlater, the oxygen concentration (oxygen partial pressure) in each of thefirst inner cavity 20 and the second inner cavity 40 can be measuredwith the reference electrode 42.

In the gas flowing portion, the gas inlet 10 provides a region opened toan external space, and the measurement object gas is taken into thesensor element 101 from the external space through the gas inlet 10. Thefirst diffusion rate-controlling portion 11 serves as a region forgiving predetermined diffusion resistance to the measurement object gasthat has been taken in through the gas inlet 10. The buffer space 12 isa space provided to introduce the measurement object gas, which has beenintroduced from the first diffusion rate-controlling portion 11, to thesecond diffusion rate-controlling portion 13. The second diffusionrate-controlling portion 13 serves as a region for giving predetermineddiffusion resistance to the measurement object gas that is introducedfrom the buffer space 12 to the first inner cavity 20. When themeasurement object gas is introduced from the outside of the sensorelement 101 to the first inner cavity 20, the measurement object gas isabruptly taken into the inside of the sensor element 101 through the gasinlet 10 due to pressure fluctuations of the measurement object gas inthe external space (i.e., due to pulsation of exhaust pressure when themeasurement object gas is exhaust gas of an automobile). At that time,the taken-in measurement object gas is not directly introduced to thefirst inner cavity 20, but it is introduced to the first inner cavity 20after fluctuations in the concentration of the measurement object gashave been settled through the first diffusion rate-controlling portion11, the buffer space 12, and the second diffusion rate-controllingportion 13. As a result, the fluctuations in the concentration of themeasurement object gas introduced to the first inner cavity 20 arereduced to a substantially negligible level. The first inner cavity 20is provided as a space for adjusting the partial pressure of oxygen inthe measurement object gas that has been introduced through the seconddiffusion rate-controlling portion 13. The oxygen partial pressure isadjusted with operation of a main pump cell 21.

The main pump cell 21 is an electrochemical pump cell constituted by aninner pump electrode 22 having a ceiling electrode portion 22 a that isdisposed on the lower surface of the second solid electrolyte layer 6 ina substantially entire region facing the first inner cavity 20, an outerpump electrode 23 disposed in a state exposed to the external space onan upper surface of the second solid electrolyte layer 6 in a regioncorresponding to the ceiling electrode portion 22 a, and the secondsolid electrolyte layer 6 sandwiched between those two electrodes.

The inner pump electrode 22 is arranged to extend over respectiveportions of the solid electrolyte layers on the upper and lower sides(i.e., the second solid electrolyte layer 6 and the first solidelectrolyte layer 4), those portions defining the first inner cavity 20,and over portions of the spacer layer 5, which portions define bothsidewalls of the first inner cavity 20. More specifically, the ceilingelectrode portion 22 a is formed on the lower surface of the secondsolid electrolyte layer 6 in a region providing a ceiling surface of thefirst inner cavity 20, and a bottom electrode portion 22 b is formed onthe upper surface of the first solid electrolyte layer 4 in a regionproviding a bottom surface of the first inner cavity 20. Furthermore,lateral electrode portions (not illustrated) are formed on sidewallsurfaces (inner surfaces) of the spacer layer 5 in regions defining boththe right and left sidewalls of the first inner cavity 20, to therebyinterconnect the ceiling electrode portion 22 a and the bottom electrodeportion 22 b. Thus, the inner pump electrode 22 is disposed in the formof a tunnel-like structure in a zone where the lateral electrodeportions are disposed.

The inner pump electrode 22 and the outer pump electrode 23 are eachformed as a porous cermet electrode (e.g., a cermet electrode made of Ptand ZrO₂ and containing 1% of Au). The inner pump electrode 22contacting the measurement object gas is made of a material having aweakened reducing ability with respect to NOx components in themeasurement object gas.

In the main pump cell 21, oxygen in the first inner cavity 20 can bepumped out to the external space, or oxygen in the external space can bepumped into the first inner cavity 20 by applying a desired pump voltageVp0 between the inner pump electrode 22 and the outer pump electrode 23,thus causing a pump current Ip0 to flow in a positive direction or anegative direction between the inner pump electrode 22 and the outerpump electrode 23.

Furthermore, to detect the oxygen concentration (oxygen partialpressure) in an atmosphere inside the first inner cavity 20, anelectrochemical sensor cell, i.e., an oxygen partial-pressure detectionsensor cell 80 for controlling a main pump, is constituted by the innerpump electrode 22, the second solid electrolyte layer 6, the spacerlayer 5, the first solid electrolyte layer 4, the third substrate layer3, and the reference electrode 42.

The oxygen concentration (oxygen partial pressure) in the first innercavity 20 can be determined by measuring an electromotive force V0 inthe oxygen partial-pressure detection sensor cell 80 for controlling themain pump. Moreover, the pump current Ip0 is controlled by feedbackcontrolling the pump voltage Vp0, which is given from a variable powersupply 25, such that the electromotive force V0 is held constant. As aresult, the oxygen concentration in the first inner cavity 20 can beheld at a predetermined constant value.

The third diffusion rate-controlling portion 30 serves as a region forapplying predetermined diffusion resistance to the measurement objectgas of which oxygen concentration (oxygen partial pressure) has beencontrolled in the first inner cavity 20 with the operation of the mainpump cell 21, and for introducing the relevant measurement object gas tothe second inner cavity 40.

The second inner cavity 40 is provided as a space used to performtreatment regarding measurement of the concentration of nitrogen oxides(NOx) in the measurement object gas that has been introduced through thethird diffusion rate-controlling portion 30. The measurement of the NOxconcentration is mainly performed with the operation of a measurementpump cell 41 in the second inner cavity 40 after the oxygenconcentration has been adjusted by an auxiliary pump cell 50 in thesecond inner cavity 40.

In the second inner cavity 40, further adjustment of the oxygen partialpressure is performed by the auxiliary pump cell 50 on the measurementobject gas that has been introduced to the second inner cavity 40through the third diffusion rate-controlling portion 30 after theadjustment of the oxygen concentration (oxygen partial pressure) in thefirst inner cavity 20. As a result, the oxygen concentration in thesecond inner cavity 40 can be held constant with high accuracy, and theconcentration of NOx can be measured with high accuracy by the gassensor 100 of this embodiment.

The auxiliary pump cell 50 is an auxiliary electrochemical pump cellconstituted by an auxiliary pump electrode 51 having a ceiling electrodeportion 51 a, which is disposed on the lower surface of the second solidelectrolyte layer 6 over a substantially entire region facing the secondinner cavity 40, the outer pump electrode 23 (note that a suitableelectrode outside the sensor element 101 can be used without beinglimited to the outer pump electrode 23), and the second solidelectrolyte layer 6.

The auxiliary pump electrode 51 is disposed inside the second innercavity 40 in the form of a tunnel-like structure, which is similar tothat of the above-mentioned inner pump electrode 22 disposed inside thefirst inner cavity 20. More specifically, the ceiling electrode portion51 a is formed in a region of the second solid electrolyte layer 6, theregion providing a ceiling surface of the second inner cavity 40, and abottom electrode portion 51 b is formed in a region of the first solidelectrolyte layer 4, the region providing a bottom surface of the secondinner cavity 40. Furthermore, lateral electrode portions (notillustrated) interconnecting the ceiling electrode portion 51 a and thebottom electrode portion 51 b are formed respectively on the right andleft wall surfaces of the spacer layer 5 in regions defining sidewallsof the second inner cavity 40. Thus, the auxiliary pump electrode 51 hasthe tunnel-like structure. Similarly to the inner pump electrode 22, theauxiliary pump electrode 51 is also made of a material having a weakenedreducing ability with respect to the NOx components in the measurementobject gas.

In the auxiliary pump cell 50, oxygen in an atmosphere inside the secondinner cavity 40 can be pumped out to the external space, or oxygen canbe pumped into the second inner cavity 40 from the external space byapplying a desired voltage viol between the auxiliary pump electrode 51and the outer pump electrode 23.

Furthermore, to control the oxygen partial pressure in the atmosphereinside the second inner cavity 40, an electrochemical sensor cell, i.e.,an oxygen partial-pressure detection sensor cell 81 for controlling anauxiliary pump, is constituted by the auxiliary pump electrode 51, thereference electrode 42, the second solid electrolyte layer 6, the spacerlayer 5, the first solid electrolyte layer 4, and the third substratelayer 3.

The auxiliary pump cell 50 performs pumping with the aid of a variablepower supply 52 of which voltage is controlled in accordance with anelectromotive force V1 that is detected by the oxygen partial-pressuredetection sensor cell 81 for controlling the auxiliary pump. As aresult, the oxygen partial pressure in the atmosphere inside the secondinner cavity 40 can be controlled to a low pressure level at which themeasurement of NOx is substantially not affected.

In addition, a pump current Ip1 from the variable power supply 52 isused to control the electromotive force of the oxygen partial-pressuredetection sensor cell 80 for controlling the main pump. Morespecifically, the pump current Ip1 is input as a control signal to theoxygen partial-pressure detection sensor cell 80 for controlling themain pump, in order to control the electromotive force V0 thereof. As aresult, a gradient of the oxygen partial pressure in the measurementobject gas introduced from the third diffusion rate-controlling portion30 to the second inner cavity 40 is controlled to be always heldconstant. When the sensor element is used as a NOx sensor, the oxygenconcentration in the second inner cavity 40 is held at a constant valueof about 0.001 ppm with the operations of the main pump cell 21 and theauxiliary pump cell 50.

The measurement pump cell 41 measures, inside the second inner cavity40, the concentration of NOx in the measurement object gas. Themeasurement pump cell 41 is an electrochemical pump cell that isconstituted by a measurement electrode 44 disposed on the upper surfaceof the first solid electrolyte layer 4 in a region facing the secondinner cavity 40 at a position spaced from the third diffusionrate-controlling portion 30, the outer pump electrode 23, the secondsolid electrolyte layer 6, the spacer layer 5, and the first solidelectrolyte layer 4.

The measurement electrode 44 is a porous cermet electrode. Themeasurement electrode 44 functions also as a NOx reducing catalyst forreducing NOx that is present in an atmosphere inside the second innercavity 40. In addition, the measurement electrode 44 is covered with afourth diffusion rate-controlling portion 45.

The fourth diffusion rate-controlling portion 45 is a film made of aceramic porous body. The fourth diffusion rate-controlling portion 45serves to restrict an amount of NOx flowing to the measurement electrode44, and it further functions as a protective film for the measurementelectrode 44. In the measurement pump cell 41, oxygen generated throughdecomposition of nitrogen oxides in an atmosphere around the measurementelectrode 44 can be pumped out, and an amount of the generated oxygencan be detected as a pump current Ip2.

To detect the oxygen partial pressure around the measurement electrode44, an electrochemical sensor cell, i.e., an oxygen partial-pressuredetection sensor cell 82 for controlling a measurement pump, isconstituted by the first solid electrolyte layer 4, the third substratelayer 3, the measurement electrode 44, and the reference electrode 42. Avariable power supply 46 is controlled in accordance with anelectromotive force V2 that is detected by the oxygen partial-pressuredetection sensor cell 82 for controlling the measurement pump.

The measurement object gas introduced to the second inner cavity 40reaches the measurement electrode 44 through the fourth diffusionrate-controlling portion 45 under the condition of the oxygen partialpressure being controlled. The nitrogen oxides in the measurement objectgas around the measurement electrode 44 are reduced (2NO→N₂+O₂), therebygenerating oxygen. The generated oxygen is subjected to pumping by themeasurement pump cell 41. At that time, a voltage Vp2 of the variablepower supply 46 is controlled such that the electromotive force V2detected by the oxygen partial-pressure detection sensor cell 82 forcontrolling the measurement pump is held constant. Because the amount ofoxygen generated around the measurement electrode 44 is in proportion tothe concentration of the nitrogen oxides in the measurement object gas,the concentration of the nitrogen oxides in the measurement object gasis calculated by employing the pump current Ip2 in the measurement pumpcell 41.

Moreover, when the measurement electrode 44, the first solid electrolytelayer 4, the third substrate layer 3, and the reference electrode 42 arecombined to constitute an electrochemical sensor cell that serves as anoxygen partial pressure detection means, it is possible to detect anelectromotive force depending on a difference between an amount ofoxygen generated with reduction of the NOx components, which are presentin the atmosphere around the measurement electrode 44, and an amount ofoxygen contained in open air as a reference. Accordingly, theconcentration of the NOx components in the measurement object gas canalso be determined from the detected electromotive force.

In addition, an electrochemical sensor cell 83 is constituted by thesecond solid electrolyte layer 6, the spacer layer 5, the first solidelectrolyte layer 4, the third substrate layer 3, the outer pumpelectrode 23, and the reference electrode 42. The oxygen partialpressure in the measurement object gas outside the gas sensor can bedetected from an electromotive force Vref that is obtained by the sensorcell 83.

In the gas sensor 100 constituted as described above, the measurementobject gas having the oxygen partial pressure, which is always held at alow constant value (i.e., a value substantially not affecting themeasurement of NOx), is applied to the measurement pump cell 41 with theoperations of the main pump cell 21 and the auxiliary pump cell 50.Thus, the concentration of NOx in the measurement object gas can bedetermined on the basis of the pump current Ip2 that flows uponpumping-out of oxygen by the measurement pump cell 41, the oxygen beinggenerated with reduction of NOx substantially in proportion to theconcentration of NOx in the measurement object gas.

In order to increase the oxygen ion conductivity of the solidelectrolyte, the sensor element 101 further includes a heater section 70with a role of temperature adjustment to heat the sensor element 101 andto hold its temperature. The heater section 70 includes a heaterconnector electrode 71, a heater 72, a through-hole 73, a heaterinsulating layer 74, and a pressure release hole 75.

The heater connector electrode 71 is an electrode formed in a statecontacting a lower surface of the first substrate layer 1. Electricpower can be supplied to the heater section 70 from the outside byconnecting the heater connector electrode 71 to an external powersupply.

The heater 72 is an electrical resistor formed in a state sandwichedbetween the second substrate layer 2 and the third substrate layer 3from below and above, respectively. The heater 72 is connected to theheater connector electrode 71 via the through-hole 73. The heater 72generates heat with supply of electric power from the outside throughthe heater connector electrode 71, thereby heating the solidelectrolytes, which constitute the sensor element 101, and holdingtemperatures thereof.

Moreover, the heater 72 is embedded in a state extending over an entireregion from the first inner cavity 20 to the second inner cavity 40 suchthat the sensor element 101 can be entirely adjusted to a temperature atwhich the above-described solid electrolytes are activated.

The heater insulating layer 74 is an insulating layer made of aninsulator, such as alumina, and formed on upper and lower surfaces ofthe heater 72. The heater insulating layer 74 is formed with intent toprovide electrical insulation between the second substrate layer 2 andthe heater 72 and electrical insulation between the third substratelayer 3 and the heater 72.

The pressure release hole 75 is formed in a state penetrating throughthe third substrate layer 3 and communicating with the reference gasintroducing space 43. The pressure release hole 75 is provided withintent to relieve a rise of inner pressure attributable to a temperaturerise within the heater insulating layer 74.

As illustrated in FIGS. 1 and 2, the element body 102 is partly coveredwith the protective layer 84. Because the sensor element 101 is in theform of a rectangular parallelepiped, it has six surfaces as outersurfaces of the solid electrolyte layers of the sensor element 101,i.e., a first surface 102 a (upper surface), a second surface 102 b(lower surface), a third surface 102 c (left lateral surface), a fourthsurface 102 d (right lateral surface), a fifth surface 102 e (front endsurface), and a sixth surface 102 f (rear end surface), as illustratedin FIGS. 1 to 3. The protective layer 84 is made of a porous material,and it includes first to fifth protective layers 84 a to 84 e that areformed respectively on five ones (first to fifth surfaces 102 a to 102e) of the six surfaces (first to sixth surfaces 102 a to 102 f) of theelement body 102. The first to fifth protective layers 84 a to 84 e arecollectively called the protective layer 84. Each of the first to fourthprotective layers 84 a to 84 d covers a region of the surface of theelement body 102 on which the relevant protective layer is formed, theregion spanning through a distance L (see FIG. 2) from the front endsurface of the element body 102 rearwards. The first protective layer 84a covers the first surface 102 a including its part where the outer pumpelectrode 23 is formed (see FIGS. 2 and 3). The fifth protective layer84 e covers the gas inlet 10 as well. However, since the fifthprotective layer 84 e is made of a porous material, the measurementobject gas can reach the gas inlet after flowing through the interior ofthe protective layer 84 e. Thus, the protective layer 84 covers aportion of the element body 102 (which portion includes the front endsurface of the element body 102 and extends through the distance L fromthe front end surface), thereby protecting the relevant portion. Inother words, the protective layer 84 serves to suppress cracking of theelement body 102, which may occur due to adherence of moisture, etc.contained in the measurement object gas, for example. The distance L isdetermined to fall within a range of (0<distance L<length of the elementbody 102 in the longitudinal direction) on the bases of a range wherethe element body 102 is exposed to the measurement object gas in the gassensor 100, the position of the outer pump electrode 23, and so on.

In this embodiment, as illustrated in FIG. 1, the element body 102 has alength in the back-forth direction, a width in the right-left direction,and a thickness in the up-down direction, which are different from oneanother and which satisfy a relation of length>width>thickness.Moreover, the distance L is set to a value larger than values of thewidth and the thickness of the element body 102.

As illustrated in FIGS. 2 to 5, a space 90 is formed at the inner sideof the protective layer 84. More specifically, the first protectivelayer 84 a includes an upper space 91, the second protective layer 84 bincludes a lower space 92, the third protective layer 84 c includes aleft space 93, and the fourth protective layer 84 d includes a rightspace 94. Those spaces 91 to 94 are collectively called the space 90.

The upper space 91 is an exposed space to which the first surface 102 ais exposed, and it is formed in a substantially rectangularparallelepiped shape. As illustrated in FIG. 4, the upper space 91 ispositioned over a range including the center of a region of the firstsurface 102 a (i.e., a region of the first surface 102 a spanning fromthe front end surface through the distance L), the region being coveredwith the protective layer 84 (first protective layer 84 a), when viewedfrom a direction perpendicular to the first surface 102 a, namely whenlooked at in a plan view. In other words, the center of a region of thefirst surface 102 a, the region being covered with the protective layer84 (first protective layer 84 a), is exposed to the upper space 91.Here, the expression “the center of a region of the first surface 102 a,the region being covered with the protective layer 84” implies themiddle of the relevant region in each of the back-forth direction andthe right-left direction. The upper space 91 may be positioned such thatat least a part of the upper space 91 overlaps at least a part of theouter pump electrode 23 when viewed from above. In this embodiment, asillustrated in FIG. 4, the upper space 91 is positioned in anoverlapping relation to the entire outer pump electrode 23 (namely,including the entire outer pump electrode 23) when viewed from above.Stated in another way, the outer pump electrode 23 is entirely exposedto the upper space 91.

The second surface 102 b, the third surface 102 c, and the fourthsurface 102 d are exposed to the lower space 92, the left space 93, andthe right space 94, respectively, unlike the upper space 91, but thosespaces 92, 93 and 94 have a similar shape to that of the upper space 91.Positional relations of the lower space 92, the left space 93, and theright space 94 relative to the second surface 102 b, the third surface102 c, and the fourth surface 102 d, respectively, are similar to thatof the upper space 91 relative to the first surface 102 a. The upperspace 91 and the lower space 92 are formed in shapes and layoutssymmetrical in the up-down direction. The left space 93 and the rightspace 94 are formed in shapes and layouts symmetrical in the right-leftdirection.

More specifically, the lower space 92 is an exposed space to which thesecond surface 102 b is exposed, and it is formed in a substantiallyrectangular parallelepiped shape. As illustrated in FIG. 5, the lowerspace 92 is positioned over a range including the center of a region ofthe second surface 102 b (i.e., a region of the second surface 102 bspanning from the front end surface through the distance L), the regionbeing covered with the protective layer 84 (second protective layer 84b), when viewed from a direction perpendicular to the second surface 102b, namely when viewed from below.

The left space 93 is a space to which the third surface 102 c isexposed, and it is positioned over a range including the center of aregion of the third surface 102 c (i.e., a region of the third surface102 c spanning from the front end surface through the distance L), theregion being covered with the protective layer 84 (third protectivelayer 84 c), when viewed from a direction perpendicular to the thirdsurface 102 c, namely when viewed from left. The right space 94 is aspace to which the fourth surface 102 d is exposed, and it is positionedover a range including the center of a region of the fourth surface 102d (i.e., a region of the fourth surface 102 d spanning from the frontend surface through the distance L), the region being covered with theprotective layer 84 (fourth protective layer 84 d), when viewed from adirection perpendicular to the fourth surface 102 d, namely when viewedfrom right.

The protective layer 84 is made of a porous material, such as an aluminaporous material, a zirconia porous material, a spinel porous material, acordierite porous material, a titania porous material, or a magnesiaporous material. In this embodiment, the protective layer 84 is made ofan alumina porous material. A film thickness of the protective layer 84is, e.g., 100 to 1000 μm, and a porosity of the protective layer 84 is,e.g., 5% to 85% by volume, though not being particularly limited tothose values.

A method of manufacturing the gas sensor 100 thus constituted will bedescribed below. According to the method of manufacturing the gas sensor100, the element body 102 is first fabricated, and the protective layer84 is then formed on the element body 102, whereby the sensor element101 is manufactured.

A method of fabricating the element body 102 is as follows. First, sixceramic green sheets, each not yet fired, are prepared. Patterns forelectrodes, insulating layers, resistance heating bodies, etc. areprinted on the individual ceramic green sheets corresponding to thefirst substrate layer 1, the second substrate layer 2, the thirdsubstrate layer 3, the first solid electrolyte layer 4, the spacer layer5, and the second solid electrolyte layer 6, respectively. After formingthe various patterns as described above, the green sheets are dried.Thereafter, the dried green sheets are successively stacked to form alaminate. The laminate thus obtained includes the plurality of elementbodies 102. The individual element bodies 102 are obtained by cuttingthe laminate into pieces per size of each element body 102, and byfiring the pieces at a predetermined temperature.

A method of forming the protective layer 84 on the element body 102 willbe described below. The protective layer 84 can be formed by suitableone of various methods, such as gel casting, screen printing, dipping,and plasma spraying. The space 90 in the protective layer 84 can beformed by employing a melt-disappearing material (e.g., carbon ortheobromine) that disappears with burning.

The gel casting is a process of solidifying slurry through a chemicalreaction of the slurry itself, thereby obtaining a shaped body. Forexample, a part of the element body 102 (i.e., its portion to becovered) is positioned in a state exposed to the interior of a shapingmold. The slurry is introduced to flow into a gap between the shapingmold and the element body 102, and is then solidified. The slurry foruse in the gel casting process may be a mixture containing raw materialpowder (e.g., ceramic particles) of the protective layer 84, a sinteringaid, an organic solvent, a dispersant, and a gelling agent. The gellingagent is not limited to particular one insofar as it contains at leasttwo kinds of polymerizable organic compounds. For example, the gellingagent contains two kinds of organic compounds capable of developing theurethane reaction. Those two kinds of organic compounds are, e.g., oneof isocyanates and one of polyols. In the preparation of the slurry, aslurry precursor is first prepared by adding the raw material powder,the sintering aid, the organic solvent, and the dispersant together at apredetermined ratio, and mixing them for a predetermined time.Immediately before using the slurry, the gelling agent is added to andmixed with the slurry precursor, whereby the slurry is obtained. As analternative, at least one or two of the isocyanate, the polyol, and acatalyst, which are components of the gelling agent, may be previouslyadded to the slurry precursor, and the remaining component(s) may bethen added at the time of preparing the slurry. The slurry obtainedafter adding the gelling agent to the slurry precursor is preferablyintroduced to quickly flow into the shaping mold because the chemicalreaction (urethane reaction) of the gelling agent starts to progresswith the lapse of time.

The protective layer 84 is formed, by way of example, as follows. First,a melt-disappearing body is formed in shape corresponding to the shapeof the space 90, i.e., to respective shapes of the upper space 91, thelower space 92, the left space 93, and the right space 94, by coating amelt-disappearing material over the surfaces of the element body 102,and by drying the coated melt-disappearing material. Themelt-disappearing material can be coated, for example, by screenprinting, gravure printing, or ink jet printing. The melt-disappearingbody may be formed by repeating the coating and drying steps pluraltimes. Then, the protective layer 84 is formed over the surfaces of theelement body 102. The protective layer 84 including themelt-disappearing body in the shape corresponding to the space 90 isthus formed. Thereafter, the melt-disappearing body is eliminated byburning. As a result, a portion occupied by the melt-disappearing bodybecomes the space 90, and the protective layer 84 including the space 90therein is formed. In such a manner, the protective layer 84 is formedon the element body 102, and the sensor element 101 is obtained. Whenthe protective layer 84 is formed by the gel casting, the screenprinting, or the dipping, the protective layer 84 is obtained bysolidifying and drying the slurry that becomes the protective layer 84,and then firing the slurry. In that case, the firing of the protectivelayer 84 and the burning of the melt-disappearing body may be performedat the same time. When the protective layer 84 is formed by the plasmaspraying, the melt-disappearing body may be burnt to disappear afterforming the protective layer 84.

In the case of forming the protective layer 84 including themelt-disappearing body in the shape corresponding to the space 90 (orthe protective layer 84 before the firing), the protective layer 84 andthe melt-disappearing body may be formed by repeating formation of apart of the protective layer 84 and a part of the melt-disappearing bodysuch that the protective layer 84 is formed in a state successivelylaminated from the surface(s) of the element body 102 in the directionof thickness thereof. The protective layer 84 may be formed in a mannerof forming the first to fifth protective layers 84 a to 84 e together,or forming the first to fifth protective layers 84 a to 84 e one by one.

After fabricating the sensor element 101 as described above, the gassensor 100 is obtained by placing the sensor element 101 into apredetermined housing, and by assembling the sensor element 101 into abody (not illustrated) of the gas sensor 100.

In use of the gas sensor 100 obtained as described above, themeasurement object gas in exhaust piping reaches the sensor element 101and flows into the gas inlet 10 after passing through the protectivelayer 84. The sensor element 101 then detects the concentration of NOxin the measurement object gas having flowed into the gas inlet 10. Atthat time, moisture contained in the measurement object gas may oftenadhere to surfaces of the protective layer 84. As described above, thetemperature of the element body 102 is adjusted by the heater 72 to atemperature (e.g., about 800° C.) at which the solid electrolytes areactivated, and upon moisture adhering to the sensor element body 102,the temperature is abruptly lowered, thus causing cracking of theelement body 102 in some cases. Here, the protective layer 84 in thisembodiment includes the space 90 at the inner side. Therefore, heatconduction in the direction of thickness of the protective layer 84(i.e., in the direction toward the element body 102 from the outerperipheral surface of the protective layer 84) can be blocked off by thespace 90, and cooling of the element body 102 can be suppressed whenmoisture adheres to the surfaces of the protective layer 84. Morespecifically, with the presence of the upper space 91, the lower space92, the left space 93, and the right space 94, cooling of the elementbody 102 can be suppressed when moisture adheres to upper, lower, leftand right surfaces of the protective layer 84. Accordingly, the moistureresistance of the element body 102 is improved in this embodiment.

Correspondence relations between components in this embodiment andcomponents in the present invention are clarified here. The element body102 in this embodiment corresponds to an element body in the presentinvention. Furthermore, the outer pump electrode 23 corresponds to anouter electrode, the upper space 91 corresponds to an exposed space, andthe protective layer 84 corresponds to a protective layer.

According to the gas sensor 100 of this embodiment described above indetail, the sensor element 101 includes the element body 102 having theelongate rectangular parallelepiped shape and including the solidelectrolyte layers with oxygen ion conductivity, the outer pumpelectrode 23 disposed on the first surface 102 a of the element body102, and the protective layer 84 covering at least a part of the firstsurface 102 a of the element body 102 and including one or more exposedspaces (such as the upper space 91) to which the first surface 102 a isexposed. With those features, since heat conduction in the direction ofthickness of the protective layer 84 (particularly heat conduction inthe direction downward from the upper surface of the first protectivelayer 84 a) can be blocked off by the upper space 91, cooling of theelement body 102 is suppressed when moisture adheres to the surface ofthe protective layer 84. Hence the moisture resistance of the elementbody 102 is improved. Moreover, since cooling of the element body 102can be suppressed with the presence of the lower space 92, the leftspace 93, and the right space 94 when moisture adheres to the second tofourth protective layers 84 b to 84 d, respectively, the moistureresistance of the element body 102 is further improved.

The upper space 91 is positioned in an overlapping relation to themiddle of a region of the first surface 102 a, the region being coveredwith the protective layer 84, when viewed from the directionperpendicular to the first surface 102 a. Here, temperature in themiddle of the region covered with the protective layer 84 iscomparatively more apt to become high, and temperature in a part of therelevant region farther away from the middle (e.g., a part nearer to anend of the first surface 102 a) is comparatively less apt to becomehigh. Therefore, the moisture resistance of the element body 102 isfurther improved by heat-insulating the middle of a region of the firstsurface 102 a, the region being covered with the protective layer 84, bythe upper space 91.

Furthermore, the upper space 91 is positioned such that at least a partof the upper space 91 overlaps at least a part of the outer pumpelectrode 23 when viewed from the direction perpendicular to the firstsurface 102 a. Here, the outer pump electrode 23 has higher thermalconductivity than the solid electrolyte layers (i.e., the layers 1 to6), and its temperature is comparatively more apt to become high.Therefore, the moisture resistance is further improved byheat-insulating at least the part of the outer pump electrode 23 by theupper space 91. In addition, the upper space 91 is positioned in arelation overlapping the entire outer pump electrode 23 (namely,including the entire outer pump electrode 23) when viewed from thedirection perpendicular to the first surface 102 a. Therefore, the outerpump electrode 23 can be entirely heat-insulated by the upper space 91.As a result, the moisture resistance of the element body 102 is furtherimproved.

Second Embodiment

FIG. 6 is a sectional view of a sensor element 101A according to asecond embodiment. FIG. 7 is a sectional view taken along E-E in FIG. 6.The sensor element 101A is similar to the sensor element 101 accordingto the first embodiment except that the protective layer 84 includes aspace 90A different from the space 90.

The space 90A includes an upper space 91A, a lower space 92A, a leftspace 93A, and a right space 94A. The upper space 91A is similar to theupper space 91 in the first embodiment except for further including acommunication hole H1 that is opened to the upper surface of the firstprotective layer 84 a. Likewise, the other spaces 92A to 94A are similarto the spaces 92 to 94 in the first embodiment, respectively, except forfurther having communication holes H2 to H4 that are opened to the outersurfaces of the protective layer 84.

The communication hole H1 is a rectangular parallelepiped space that isopened in a substantially rectangular shape to the upper surface of theprotective layer 84. However, the present invention is not limited tothe above case, and the communication hole H1 may be opened in acircular shape, for example. The upper space 91A is communicated withthe outside of the protective layer 84 via the communication hole H1.The communication hole H1 may be positioned, when viewed from thedirection perpendicular to the first surface 102 a, at the middle of aregion of the first surface 102 a in the back-forth direction, theregion being covered with the protective layer 84, at the middle of therelevant region in the right-left direction, or at the center of therelevant region (i.e., at the middle of the relevant region in each ofthe back-forth direction and the right-left direction). Moreover, thecommunication hole H1 may be formed at a position overlapping the outerpump electrode 23 when viewed from the direction perpendicular to thefirst surface 102 a. In this embodiment, the communication hole H1 isformed at a position that is the center of a region of the first surface102 a, the region being covered with the protective layer 84, and thatoverlaps the outer pump electrode 23 when viewed from the directionperpendicular to the first surface 102 a.

Positional relations of the communication holes H2 to H4 relative to thecorresponding second to fourth surfaces 102 b to 102 d, respectively,and shapes thereof are similar to the positional relation of thecommunication hole H1 relative to the first surface 102 a and the shapethereof. In this embodiment, the upper space 91A and the lower space 92Aare formed in shapes and layouts symmetrical in the up-down direction.The left space 93A and the right space 94A are formed in shapes andlayouts symmetrical in the right-left direction.

Also in the sensor element 101A described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the spaces 91A to 94A. Moreover, the upperspace 91A is provided with an opening (opening of the communication holeH1) in communication with the outside of the first protective layer 84a. With the presence of the opening, since heat in the upper space 91Acan be released through the opening, overheating of the element body 102is suppressed. As a result, an abrupt temperature drop of the elementbody 102 can be suppressed when moisture adheres to the surfaces of theprotective layer 84, and the moisture resistance of the element body 102is improved. Likewise, since the spaces 92A to 94A are provided withrespective openings of the communication holes H2 to H4, the moistureresistance of the element body 102 is improved.

Third Embodiment

FIG. 8 is a sectional view of a sensor element 101B according to a thirdembodiment. FIG. 9 is a sectional view taken along F-F in FIG. 8. Thesensor element 101B is similar to the sensor element 101 according tothe first embodiment except that the protective layer 84 includes aspace 90B different from the space 90.

The space 90B includes an upper space 91B, a lower space 92B, a leftspace 93B, and a right space 94B. The upper space 91B includes aplurality of exposed spaces 95B1 to which the first surface 102 a isexposed, and a plurality of non-exposed spaces 96B1 to which the firstsurface 102 a is not exposed, and which are present at positionsdeviated from the exposed spaces 95B1 in the direction perpendicular tothe first surface 102 a (upward). The exposed spaces 95B1 are each asubstantially rectangular parallelepiped space. As illustrated in FIG.9, there are twelve exposed spaces 95B1 in total, namely three in theright-left direction and four in the back-forth direction, which arearrayed in a lattice pattern when viewed from above. The non-exposedspaces 96B1 are each a substantially rectangular parallelepiped space.As illustrated in FIG. 9, there are twenty non-exposed spaces 96B1 intotal, namely four in the right-left direction and five in theback-forth direction, which are arrayed in a lattice pattern when viewedfrom above. All the exposed spaces 95B1 have the same length in theup-down direction. All the non-exposed spaces 96B1 have the same lengthin the up-down direction and are positioned at the same level in theup-down direction. Thus, the exposed spaces 95B1 and the non-exposedspaces 96B1 are arrayed in two upper and lower stages.

The exposed spaces 95B1 and the non-exposed spaces 96B1 are present atdifferent heights in the up-down direction such that their positions donot overlap with each other when viewed from the direction perpendicularto the up-down direction. Furthermore, the exposed spaces 95B1 and theexposed spaces 96B1 are present at positions deviated from each other inthe right-left direction and deviated from each other in the back-forthdirection.

The lower space 92B includes a plurality of exposed spaces 95B2 to whichthe second surface 102 b is exposed, and a plurality of non-exposedspaces 96B2 to which the second surface 102 b is not exposed, and whichare present at positions deviated from the exposed spaces 95B2 in thedirection perpendicular to the second surface 102 b (downward). The leftspace 93B includes a plurality of exposed spaces 95B3 to which the thirdsurface 102 c is exposed, and a plurality of non-exposed spaces 96B3 towhich the third surface 102 c is not exposed, and which are present atpositions deviated from the exposed spaces 95B3 in the directionperpendicular to the third surface 102 c (leftward). The right space 94Bincludes a plurality of exposed spaces 95B4 to which the fourth surface102 d is exposed, and a plurality of non-exposed spaces 96B4 to whichthe fourth surface 102 d is not exposed, and which are present atpositions deviated from the exposed spaces 95B4 in the directionperpendicular to the fourth surface 102 d (rightward). Positionalrelations of the exposed spaces 95B2 to 95B4 and the non-exposed spaces96B2 to 96B4 relative to the corresponding second to fourth surfaces 102b to 102 d, respectively, and shapes thereof are similar to thepositional relations of the exposed spaces 95B1 and the non-exposedspaces 96B1 relative to the first surface 102 a and the shapes thereof.In this embodiment, the exposed spaces 95B3 and 95B4 are each formed onein the up-down direction and four in the back-forth direction, i.e.,four in total. The non-exposed spaces 96B3 and 96B4 are each formed twoin the up-down direction and five in the back-forth direction, i.e., tenin total. Moreover, in this embodiment, the upper space 91B and thelower space 92B are formed in shapes and layouts symmetrical in theup-down direction. The left space 93B and the right space 94B are formedin shapes and layouts symmetrical in the right-left direction.

Also in the sensor element 101B described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the spaces 91B to 94B. Moreover, the firstprotective layer 84 a includes the plurality of exposed spaces 95B1, andthe plurality of non-exposed spaces 96B1 to which the first surface 102a is not exposed, and which are present at positions deviated from atleast one of the exposed spaces 95B1 in the direction perpendicular tothe first surface 102 a. When the plural spaces are present in the firstprotective layer 84 a as described above, the strength of the firstprotective layer 84 a is less apt to reduce in comparison with the casewhere one space having the same total volume is present in the firstprotective layer 84 a. Therefore, it is possible to improve the moistureresistance of the element body 102 with the provision of the spaces inthe first protective layer 84 a, and to suppress the reduction in thestrength of the first protective layer 84 a, which may occur with thepresence of the spaces. Moreover, since the non-exposed spaces 96B1 arefurther present at positions deviated from the exposed spaces 95B1 inthe vertical direction, the positions of the spaces can be more easilyarranged farther apart from each other than in the case where the sameplural number of spaces having the same shape are all arranged atpositions deviated in a direction parallel to the first surface 102 a(without being deviated in the vertical direction). As a result, thereduction in the strength of the first protective layer 84 a can befurther suppressed. With respect to the other spaces 92B to 94B, similaradvantageous effects can be obtained with similar constitutions to thatdescribed above.

Fourth Embodiment

FIG. 10 is a sectional view of a sensor element 101C according to afourth embodiment. FIG. 11 is a sectional view taken along G-G in FIG.10. The sensor element 101C is similar to the sensor element 101Baccording to the third embodiment except that the protective layer 84includes a space 90C different from the space 90B.

The space 90C includes an upper space 91C, a lower space 92C, a leftspace 93C, and a right space 94C. The upper space 91C includes aplurality of exposed spaces 95C1 to which the first surface 102 a isexposed, and a plurality of non-exposed spaces 96C1 to which the firstsurface 102 a is not exposed, and which are present at positionsdeviated from the exposed spaces 95C1 in the direction perpendicular tothe first surface 102 a (upward). Similarly, the spaces 92C to 94Cinclude plural exposed spaces 95C2 to 95C4 and plural non-exposed spaces96C2 to 96C4, respectively.

The number, the shape, and the layout of the non-exposed spaces 96C1 aresimilar to those of the non-exposed spaces 96B1 in the third embodiment.The numbers, the shapes, and the layouts of the non-exposed spaces 96C2to 96C4 are also similar to those of the non-exposed spaces 96B2 to 96B4in the third embodiment, respectively.

The exposed spaces 95C1 and the non-exposed spaces 96C1 are present atpositions deviated from each other in the direction perpendicular to thefirst surface 102 a and in the back-forth direction, but they arepresent at the same positions in the right-left direction. This point isdifferent from the positional relation between the exposed spaces 95B1and the non-exposed spaces 96B1 in the third embodiment. As illustratedin FIG. 11, there are sixteen exposed spaces 95C1 in total, namely fourin the right-left direction and four in the back-forth direction, whichare arrayed in a lattice pattern when viewed from above.

Likewise, the other exposed spaces 95C2 to 95C4 are present at the samepositions as the corresponding non-exposed spaces 96C2 to 96C4 in theright-left direction, respectively. Positional relations of the exposedspaces 95C2 to 95C4 relative to the corresponding second to fourthsurfaces 102 b to 102 d, respectively, and shapes thereof are similar tothe positional relation of the exposed spaces 95C1 relative to the firstsurface 102 a and the shape thereof. In this embodiment, the exposedspaces 95C3 and 95C4 are each formed two in the up-down direction andfour in the back-forth direction, i.e., eight in total. Furthermore, inthis embodiment, the upper space 91C and the lower space 92C are formedin shapes and layouts symmetrical in the up-down direction. The leftspace 93C and the right space 94C are formed in shapes and layoutssymmetrical in the right-left direction.

The sensor element 101C described above can also provide similaradvantageous effects to those of the sensor element 101 and the sensorelement 101B with similar features.

Fifth Embodiment

FIG. 12 is a sectional view of a sensor element 101D according to afifth embodiment. FIG. 13 is a sectional view taken along H-H in FIG.12. The sensor element 101D is similar to the sensor element 101according to the first embodiment except that the protective layer 84includes a space 90D different from the space 90.

The space 90D includes an upper space 91D, a lower space 92D, a leftspace 93D, and a right space 94D. The upper space 91D is similar to theupper space 91 in the first embodiment except that a plurality of pillarportions P1 are formed to hold the upper space 91D in the directionperpendicular to the first surface 102 a. Likewise, the other spaces 92Dto 94D are also similar to the upper spaces 92 to 94 in the firstembodiment, respectively, except that sets of plural pillar portions P2to P4 are formed to hold the corresponding spaces, respectively, in thedirections perpendicular to the corresponding second to fourth surfaces102 b to 102 d.

Each of the pillar portions P1 is a part of the first protective layer84 a and has a rectangular pillar shape. Without being limited to therectangular pillar shape, the pillar portion P1 may have the shape of acircular pillar, a triangular pillar, or a pentagonal or more polygonalpillar. As illustrated in FIG. 13, the pillar portions P1 are arrangedwith a tendency that a density of the pillar portions P1 graduallyincreases from the middle of a region of the first surface 102 a in theback-forth direction, the region being covered with the protective layer84, toward a position farther away from the middle when viewed from thedirection perpendicular to the first surface 102 a. More specifically,the pillar portions P1 are arranged with a tendency that the number ofthe pillar portions P1 per unit area gradually increases (namely, thepillar portions P1 are present more closely) toward a position fartheraway from the above-mentioned middle in the back-forth direction. Statedin another way, the pillar portions P1 are arranged with a tendency thatthe number of the pillar portions P1 per unit area gradually increasestoward a front end or a rear end of a region of the first surface 102 a,the region being covered with the protective layer 84. Furthermore, thepillar portions P1 are arranged with a tendency that the number of thepillar portions P1 per unit area gradually increases toward a positionfarther away from the outer pump electrode 23 when viewed from thedirection perpendicular to the first surface 102 a. Moreover, the pillarportions P1 are arranged with a tendency that the number of the pillarportions P1 per unit area is larger at a position not overlapping theouter pump electrode 23 than at a position overlapping the outer pumpelectrode 23 when viewed from the direction perpendicular to the firstsurface 102 a.

Positional relations of the pillar portions P2 to P4 relative to thecorresponding second to fourth surfaces 102 b to 102 d, respectively,and shapes thereof are similar to the positional relation of the pillarportions P1 relative to the first surface 102 a and the shape thereof.For example, the pillar portions P2 are arranged with a tendency thatthe number of the pillar portions P2 per unit area gradually increasesfrom the middle of a region of the second surface 102 b in theback-forth direction, the region being covered with the protective layer84, toward a position farther away from the middle when viewed from thedirection perpendicular to the second surface 102 b. The pillar portionsP3 are arranged with a tendency that the number of the pillar portionsP3 per unit area gradually increases from the middle of a region of thethird surface 102 c in the back-forth direction, the region beingcovered with the protective layer 84, toward a position farther awayfrom the middle when viewed from the direction perpendicular to thethird surface 102 c. The pillar portions P4 are arranged with a tendencythat the number of the pillar portions P4 per unit area graduallyincreases from the middle of a region of the fourth surface 102 d in theback-forth direction, the region being covered with the protective layer84, toward a position farther away from the middle when viewed from thedirection perpendicular to the fourth surface 102 d. Moreover, in thisembodiment, the pillar portions P1 and the pillar portions P2 are formedin shapes and layouts symmetrical in the up-down direction. The pillarportions P3 and the pillar portions P4 are formed in shapes and layoutssymmetrical in the right-left direction.

Also in the sensor element 101D described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the spaces 91D to 94D. Moreover, lookingat the upper space 91D, the first protective layer 84 a includes one ormore pillar portions P1 holding the upper space 91D in the directionperpendicular to the first surface 102 a. Thus, since the pillarportions P1 hold the upper space 91D, the reduction in the strength ofthe first protective layer 84 a can be suppressed. In addition, thefirst protective layer 84 a includes the plurality of pillar portionsP1, and these pillar portions P1 are arranged with a tendency that adensity of the pillar portions P1 gradually increases from the middle ofa region of the first surface 102 a, the region being covered with thefirst protective layer 84 a, toward a position farther away from themiddle. Here, temperature in the middle of the region covered with thefirst protective layer 84 a is comparatively more apt to become high,and temperature in a zone farther away from the middle (e.g., a zonenearer to an end of the first surface 102 a) is comparatively less aptto become high. Therefore, by arranging the pillar portions P1 at ahigher density in a zone where temperature is comparatively less apt tobecome high, reduction in the heat insulation effect of the upper space91D attributable to the presence of the pillar portions P1 can besuppressed in the region where temperature is more apt to become high,while the reduction in the strength of the first protective layer 84 ais suppressed by the pillar portions P1. It is hence possible to notonly further improve the moisture resistance of the element body 102,but also further suppress the reduction in the strength of the firstprotective layer 84 a. With respect to the other spaces 92D to 94D,similar advantageous effects can be obtained with similar constitutionsto that described above.

Furthermore, the pillar portions P1 are arranged with a tendency that adensity of the pillar portions P1 gradually increases toward a positionfarther away from the outer pump electrode 23 when viewed from thedirection perpendicular to the first surface 102 a. Here, the outer pumpelectrode 23 has higher thermal conductivity than the solid electrolytelayers (i.e., the layers 1 to 6), and its temperature is more apt tobecome higher than a region of the first surface 102 a where the outerpump electrode 23 is not disposed. Therefore, by arranging the pillarportions P1 at a higher density in the zone farther away from the outerpump electrode 23 where temperature is comparatively less apt to becomehigh, the reduction in the heat insulation effect of the upper space 91Dattributable to the presence of the pillar portions P1 can be suppressedin a zone including the outer pump electrode 23 where temperature ismore apt to become high, while the reduction in the strength of thefirst protective layer 84 a is suppressed by the pillar portions P1. Itis hence possible to not only further improve the moisture resistance ofthe element body 102, but also further suppress the reduction in thestrength of the first protective layer 84 a.

Sixth Embodiment

FIG. 14 is a sectional view of a sensor element 101E according to asixth embodiment. FIG. 15 is a sectional view taken along I-I in FIG.14. The sensor element 101E is similar to the sensor element 101Daccording to the fifth embodiment except that the protective layer 84includes a space 90E different from the space 90D.

The space 90E includes an upper space 91E, a lower space 92E, a leftspace 93E, and a right space 94E. The spaces 91E to 94E are similar tothe spaces 91D to 94D in the fifth embodiment, respectively, except thatthe layouts and the numbers of the pillar portions P1 to P4 for holdingthe spaces 91E to 94E are different.

As illustrated in FIG. 15, the pillar portions P1 are arranged with atendency that a density of the pillar portions P1 gradually increases ata position nearer to the middle of a region of the first surface 102 ain the back-forth direction, the region being covered with theprotective layer 84, when viewed from the direction perpendicular to thefirst surface 102 a. More specifically, the pillar portions P1 arearranged with a tendency that the number of the pillar portions P1 perunit area gradually increases (namely, the pillar portions P1 arepresent more closely) at a position nearer to the middle in theback-forth direction. Furthermore, the pillar portions P1 are arrangedwith a tendency that the number of the pillar portions P1 per unit areagradually increases at a position nearer to the outer pump electrode 23when viewed from the direction perpendicular to the first surface 102 a.Moreover, the pillar portions P1 are arranged with a tendency that thenumber of the pillar portions P1 per unit area is larger at a positionoverlapping the outer pump electrode 23 than at a position notoverlapping the outer pump electrode 23 when viewed from the directionperpendicular to the first surface 102 a.

Positional relations of the pillar portions P2 to P4 relative to thecorresponding second to fourth surfaces 102 b to 102 d, respectively,and shapes thereof are similar to the positional relation of the pillarportions P1 relative to the first surface 102 a and the shape thereof.For example, the pillar portions P2 are arranged with a tendency thatthe number of the pillar portions P2 per unit area gradually increasesat a position nearer to the middle of a region of the second surface 102b in the back-forth direction, the region being covered with theprotective layer 84, when viewed fruit the direction perpendicular tothe second surface 102 b. The pillar portions P3 are arranged with atendency that the number of the pillar portions P3 per unit areagradually increases at a position nearer to the middle of a region ofthe third surface 102 c in the back-forth direction, the region beingcovered with the protective layer 84, when viewed fruit the directionperpendicular to the third surface 102 c. The pillar portions P4 arearranged with a tendency that the number of the pillar portions P4 perunit area gradually increases at a position nearer to the middle of aregion of the fourth surface 102 d in the back-forth direction, theregion being covered with the protective layer 84, when viewed from thedirection perpendicular to the fourth surface 102 d. Moreover, in thisembodiment, the pillar portions P1 and the pillar portions P2 are formedin shapes and layouts symmetrical in the up-down direction. The pillarportions P3 and the pillar portions P4 are formed in shapes and layoutssymmetrical in the right-left direction.

Also in the sensor element 101E described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the spaces 91E to 94E. Moreover, since thepillar portions P1 hold the upper space 91E, the reduction in thestrength of the first protective layer 84 a can be suppressed. Withrespect to the other spaces 92D to 94D, similar advantageous effects canbe obtained with similar constitutions to that described above.

Seventh Embodiment

FIG. 16 is a sectional view of a sensor element 101F according to aseventh embodiment. FIG. 17 is a sectional view taken along J-J in FIG.16. FIG. 18 is a sectional view taken along K-K in FIG. 16. The sensorelement 101F is similar to the sensor element 101 according to the firstembodiment except that the protective layer 84 includes a space 90Fdifferent from the space 90.

The space 90F includes an upper space 91F, a lower space 92F, a leftspace 93F, and a right space 94F. As illustrated in FIG. 17, the upperspace 91F includes a plurality of first exposed spaces 97F1 that areexposed spaces each having a longitudinal direction aligned with thelongitudinal direction of the first surface 102 a (i.e., with theback-forth direction), and that are disposed side by side along theshort-length direction of the first surface 102 a (i.e., in theright-left direction). The upper space 91F further includes a pluralityof second exposed spaces 98F1 that are exposed spaces each having alongitudinal direction aligned with the short-length direction of thefirst surface 102 a and intersecting the first exposed spaces 97F1, andthat are disposed side by side along the longitudinal direction of thefirst surface 102 a. In this embodiment, there are two first exposedspaces 97F1, and five second exposed spaces 98F1. The second exposedspaces 98F1 are positioned at even intervals in the back-forthdirection.

The first exposed spaces 97F1 and the second exposed spaces 98F1 areeach a space having a substantially rectangular parallelepiped shape.Each first exposed space 97F1 has a slit-like shape and is formed suchthat one side of the first exposed space 97F1 along the short-lengthdirection of the first surface 102 a (i.e., one side extending in theright-left direction) is shorter than the other two sides (i.e., sidesextending in the up-down direction and the back-forth direction). Eachsecond exposed space 98F1 has a slit-like shape and is formed such thatone side of the second exposed space 98F1 along the longitudinaldirection of the first surface 102 a (i.e., one side extending in theback-forth direction) is shorter than the other two sides (i.e., sidesextending in the up-down direction and the right-left direction).

The lower space 92F includes, similarly to the upper space 91F, aplurality of first exposed spaces 97F2 and a plurality of second exposedspaces 98F2. The left space 93F includes, similarly to the upper space91F, a plurality of first exposed spaces 97F3 and a plurality of secondexposed spaces 98F3. The right space 94F includes, similarly to theupper space 91F, a plurality of first exposed spaces 97F4 and aplurality of second exposed spaces 98F4.

Positional relations of the first exposed spaces 97F2 to 97F4 and thesecond exposed spaces 98F2 to 98F4 relative to the corresponding secondto fourth surfaces 102 b to 102 d, respectively, and shapes thereof aresimilar to the positional relations of the first exposed spaces 97F1 andthe second exposed spaces 97F1 relative to the first surface 102 a andthe shapes thereof. For example, the first exposed spaces 97F2 to 97F4have longitudinal directions aligned respectively with the longitudinaldirections of the corresponding second to fourth surfaces 102 b to 102 d(i.e., with the back-forth direction). The second exposed spaces 98F2 to98F4 have longitudinal directions aligned respectively with theshort-length directions of the corresponding second to fourth surfaces102 b to 102 d. Moreover, the first exposed spaces 97F2 intersect thesecond exposed spaces 98F2, the first exposed spaces 97F3 intersect thesecond exposed spaces 98F3, and the first exposed spaces 97F4 intersectthe second exposed spaces 98F4. In this embodiment, the upper space 91Fand the lower space 92F are formed in shapes and layouts symmetrical inthe up-down direction. The left space 93F and the right space 94F areformed in shapes and layouts symmetrical in the right-left direction.

Also in the sensor element 101F described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the spaces 91F to 94F. Moreover, the firstprotective layer 84 a includes the plurality of first exposed spaces97F1 and the plurality of second exposed spaces 98F1. Since the pluralfirst exposed spaces 97F1 being elongate in the longitudinal directionof the first surface 102 a are present side by side along theshort-length direction of the first surface 102 a, stress generated, dueto a difference in thermal expansion coefficient between the firstprotective layer 84 a and the element body 102 when exposed to moisture,in the short-length direction of the first surface 102 a and appliedfrom the first protective layer 84 a to the element body 102 can bereduced. Furthermore, since the plural second exposed spaces 98F1 beingelongate in the short-length direction of the first surface 102 a arepresent side by side along the longitudinal direction of the firstsurface 102 a, stress generated, due to the difference in thermalexpansion coefficient between the first protective layer 84 a and theelement body 102 when exposed to moisture, in the longitudinal directionof the first surface 102 a and applied from the first protective layer84 a to the element body 102 can be reduced. As a result, the elementbody 102 is less susceptible to cracking when exposed to moisture, andthe moisture resistance of the element body 102 is further improved.With the presence of the spaces 92F to 94F, the second to fourthprotective layers 84 b to 84 d can also provide similar advantageouseffects with similar constitutions to that described above.

Eighth Embodiment

FIG. 19 is a sectional view of a sensor element 101G according to aneighth embodiment. FIG. 20 is a sectional view taken along L-L in FIG.19. In the sensor element 101G, the protective layer 84 includes a space90G. The space 90G includes an upper space 91G, a lower space 92G, aleft space 93G, and a right space 94G. The upper space 91G includes theplurality of first exposed spaces 97F1 and is similar to the upper space91F in the seventh embodiment except for not including the secondexposed spaces 98F1. Likewise, the other spaces 92G to 94G include theplural first exposed spaces 97F2 to 97F4, respectively, and they aresimilar to the spaces 92F to 94F in the seventh embodiment except fornot including the second exposed spaces 98F2 to 98F4, respectively.

Also in the sensor element 101G described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the spaces 91G to 94G. Moreover, since thefirst protective layer 84 a includes the plurality of first exposedspaces 97F1, stress generated, due to the difference in thermalexpansion coefficient between the first protective layer 84 a and theelement body 102 when exposed to moisture, in the short-length directionof the first surface 102 a and applied from the first protective layer84 a to the element body 102 can be reduced as in the seventhembodiment. As a result, the element body 102 is less susceptible tocracking when exposed to moisture, and the moisture resistance of theelement body 102 is further improved. With the presence of the firstexposed spaces 97F2 to 97F4, the second to fourth protective layers 84 bto 84 d can also provide similar advantageous effects with similarconstitutions to that described above.

Ninth Embodiment

FIG. 21 is a sectional view of a sensor element 101H according to aninth embodiment. FIG. 22 is a sectional view taken along M-M in FIG.21. In the sensor element 101H, the protective layer 84 includes a space90H. The space 90H includes an upper space 91H, a lower space 92H, aleft space 93H, and a right space 94H. The upper space 91H includes theplurality of second exposed spaces 98F1 and is similar to the upperspace 91F in the seventh embodiment except for not including the firstexposed spaces 97F1. Likewise, the other spaces 92H to 94H include theplural second exposed spaces 98F2 to 98F4, respectively, and they aresimilar to the spaces 92F to 94F in the seventh embodiment except fornot including the first exposed spaces 97F2 to 97F4, respectively.

Also in the sensor element 101H described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the spaces 91H to 94H. Moreover, since thefirst protective layer 84 a includes the plurality of second exposedspaces 98F1, stress generated, due to the difference in thermalexpansion coefficient between the first protective layer 84 a and theelement body 102 when exposed to moisture, in the longitudinal directionof the first surface 102 a and applied from the first protective layer84 a to the element body 102 can be reduced as in the seventhembodiment. As a result, the element body 102 is less susceptible tocracking when exposed to moisture, and the moisture resistance of theelement body 102 is further improved. With the presence of the secondexposed spaces 98F2 to 98F4, the second to fourth protective layers 84 bto 84 d can also provide similar advantageous effects with similarconstitutions to that described above.

Tenth Embodiment

FIG. 23 is a sectional view of a sensor element 101I according to atenth embodiment. FIG. 24 is a plan view of a part of the sensor element101I near its front end. The sensor element 101I is similar to thesensor element 101 according to the first embodiment except that theprotective layer 84 includes a space 90I different from the space 90.

The space 90I includes an upper space 91I, a lower space 92I, a leftspace 93I, and a right space 94I, which are each present plural. Theupper spaces 91I are each an exposed space to which the first surface102 a is exposed. Each upper space 91I is a space having a substantiallytriangular pillar shape and having a longitudinal direction aligned withthe longitudinal direction of the first surface 102 a. The plural upperspaces 91I (six in this embodiment) are arranged side by side along theshort-length direction of the first surface 102 a. As illustrated inFIG. 23, each upper space 91I has a triangular shape in a cross-sectionperpendicular to the first surface 102 a (i.e., in a cross-section takenalong the up-down direction and the right-left direction). Thus, theupper space 91I has a shape providing a space that gradually narrowstoward a position farther away from the first surface 102 a, i.e.,toward the upper side. Furthermore, the upper space 91I has innersurfaces Sa1 and Sb1 defining two sides of a triangle, which confrontthe first surface 102 a when viewed in the cross-section perpendicularto the first surface 102 a. The inner surfaces Sa1 and Sb1 are inclinedin such directions that they come closer to each other at a positionfarther away from the first surface 102 a, i.e., toward the upper side.Moreover, the outer pump electrode 23 is exposed to four upper spaces91I among the plural upper spaces 91, the four being positioned in acentral zone in the right-left direction.

Like the upper space 91I, the other spaces 92I to 94I are also exposedspaces to which the second to fourth surfaces 102 b to 102 d areexposed, respectively. Furthermore, the spaces 92I to 94I have innersurfaces Sa2 to Sa4 and Sb2 to Sb4, the paired inner surfaces definingtwo sides of a triangle, which confront the corresponding second tofourth surface 102 b to 104 d when viewed in the cross-sectionsperpendicular to the second to fourth surfaces, respectively. Positionalrelations of the spaces 92I to 94I relative to the corresponding secondto fourth surfaces 102 b to 102 d, respectively, and shapes thereof aresimilar to the positional relation of the upper space 91I relative tothe first surface 102 a and the shape thereof. Moreover, there are twoleft spaces 93I and two right spaces 94I arranged side by side along theup-down direction. In this embodiment, the upper spaces 91I and thelower spaces 92I are formed in shapes and layouts symmetrical in theup-down direction. The left spaces 93I and the right spaces 94I areformed in shapes and layouts symmetrical in the up-down direction.

Also in the sensor element 101I described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the spaces 91I to 94I. Moreover, the upperspace 91I has the shape providing the space that gradually narrowstoward a position farther away from the first surface 102 a. With theupper space 91I having the above-mentioned shape, the reduction in thestrength of the first protective layer 84 a can be suppressed incomparison with, for example, a rectangular parallelepiped space havingan inner surface parallel to the first surface 102 a, like the upperspace 91 in FIG. 3.

Furthermore, the upper space 91I has at least two inner surfaces Sa1 andSb1 inclined in such directions that they come closer to each other at aposition farther away from the first surface 102 a. With the upper space91I having those inner surfaces Sa1 and Sb1, the reduction in thestrength of the first protective layer 84 a can be suppressed incomparison with, for example, a rectangular parallelepiped space havingan inner surface parallel to the first surface 102 a, like the upperspace 91 in FIG. 3.

In addition, the first protective layer 84 a includes the plurality ofupper spaces 91I each of which has the longitudinal direction alignedwith the longitudinal direction of the first surface 102 a, and whichare arranged side by side along the short-length direction of the firstsurface 102 a. Therefore, stress generated, due to the difference inthermal expansion coefficient between the first protective layer 84 aand the element body 102 when exposed to moisture, in the short-lengthdirection of the first surface 102 a and applied from the firstprotective layer 84 a to the element body 102 can be reduced as in thesensor elements 101F and 101G. As a result, the element body 102 is lesssusceptible to cracking when exposed to moisture, and the moistureresistance of the element body 102 is further improved.

With the presence of the spaces 92I to 94I, the second to fourthprotective layers 84 b to 84 d can also provide similar advantageouseffects with similar constitutions to that of the first protective layer84 a.

Eleventh Embodiment

FIG. 25 is a sectional view of a sensor element 101J according to aneleventh embodiment. FIG. 26 is a plan view of the sensor element 101Jnear its front end. The sensor element 101J is similar to the sensorelement 101 according to the first embodiment except that the protectivelayer 84 includes a space 90J different from the space 90. The space 90Jincludes exposed spaces 95J1 to 95J6.

The exposed space 95J1 is a space arranged on the upper side of theelement body 102, and the first surface 102 a is exposed to the exposedspace 95J1. The exposed space 95J1 is a space having a semi-ellipticpillar shape, and an inner surface of the exposed space 95J1 confrontingthe first surface 102 a (i.e., an inner downward surface of theprotective layer 84) is formed as a curved surface (corresponding to apart of an inner peripheral surface of a cylinder), which is obtained bybending a rectangle into a shape projecting toward the outside of thefirst protective layer 84 a (i.e., upward). Thus, the exposed space 95J1has a semi-elliptic shape in a cross-section perpendicular to the firstsurface 102 a (i.e., in a cross-section taken along the up-downdirection and the right-left direction), and hence has a shape providinga space that gradually narrows toward a position farther away from thefirst surface 102 a (i.e., toward the upper side). The outer pumpelectrode 23 is entirely exposed to the exposed space 95J1. Alongitudinal direction of the exposed space 95J1 is aligned with thelongitudinal direction of the first surface 102 a.

The exposed space 95J2 is a space arranged on the lower side of theelement body 102, and the second surface 102 b is exposed to the exposedspace 95J2. The exposed space 95J2 is a space having a semi-ellipticpillar shape, and an inner surface of the exposed space 95J2 confrontingthe second surface 102 b (i.e., an inner upward surface of theprotective layer 84) is formed as a curved surface (corresponding to apart of an inner peripheral surface of a cylinder), which is obtained bybending a rectangle into a shape projecting toward the outside of thesecond protective layer 84 b (i.e., downward). A longitudinal directionof the exposed space 95J2 is aligned with the longitudinal direction ofthe second surface 102 b. The exposed space 95J2 is symmetrical in shapeand layout to the exposed space 95J1 in the up-down direction.

The exposed space 95J3 is a space arranged on the upper left side of theelement body 102, and the first surface 102 a and the third surface 102c are exposed to the exposed space 95J3. The exposed space 95J3 has ashape obtained by cutting away a part of a circular pillar. An innersurface of the exposed space 95J3 confronting the first surface 102 a(i.e., a downward surface thereof) is formed as a curved surface(corresponding to a part of an inner peripheral surface of a cylinder),which is obtained by bending a rectangle into a shape projecting towardthe outside of the first protective layer 84 a (i.e., upward). Thus, theexposed space 95J3 has a shape providing a space that gradually narrowstoward a position farther away from the first surface 102 a (i.e.,toward the upper side). An inner surface of the exposed space 95J3confronting the third surface 102 c (i.e., a rightward surface thereof)is formed as a curved surface (corresponding to a part of an innerperipheral surface of a cylinder), which is obtained by bending arectangle into a shape projecting toward the outside of the thirdprotective layer 84 c (i.e., leftward). Thus, the exposed space 95J3 hasa shape providing a space that gradually narrows toward a positionfarther away from the third surface 102 c (i.e., toward the left side).A longitudinal direction of the exposed space 95J3 is aligned with thelongitudinal direction of each of the first surface 102 a and the thirdsurface 102 c.

The exposed space 95J4 is a space arranged on the lower left side of theelement body 102, and the second surface 102 b and the third surface 102c are exposed to the exposed space 95J4. The exposed space 95J4 issymmetrical in shape and layout to the exposed space 95J3 in the up-downdirection. The exposed space 95J5 is a space arranged on the upper rightside of the element body 102, and the first surface 102 a and the fourthsurface 102 d are exposed to the exposed space 95J5. The exposed space95J5 is symmetrical in shape and layout to the exposed space 95J3 in theright-left direction. The exposed space 95J6 is a space arranged on thelower right side of the element body 102, and the second surface 102 band the fourth surface 102 d are exposed to the exposed space 95J6. Theexposed space 95J6 is symmetrical in shape and layout to the exposedspace 95J4 in the right-left direction and to the exposed space 95J5 inthe up-down direction.

The exposed spaces 95J1, 95J3 and 95J5 are arranged side by side alongthe short-length direction of the first surface 102 a. The exposedspaces 95J2, 95J4 and 95J6 are arranged side by side along theshort-length direction of the second surface 102 b. The exposed spaces95J3 and 95J4 are arranged side by side along the short-length directionof the third surface 102 c. The exposed spaces 95J5 and 95J6 arearranged side by side along the short-length direction of the fourthsurface 102 d.

Also in the sensor element 101J described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the exposed spaces 95J1 to 95J6. Moreover,each of the exposed spaces 95J1, 95J3 and 95J5 has the shape providingthe space that gradually narrows toward a position farther away from thefirst surface 102 a. In addition, the inner surfaces of the exposedspaces 95J1, 95J3 and 95J5 confronting the first surface 102 a areformed as the curved surfaces projecting outward. With those spaces eachhaving the above-mentioned shape, the reduction in the strength of thefirst protective layer 84 a can be suppressed in comparison with, forexample, a rectangular parallelepiped space having an inner surfaceparallel to the first surface 102 a, like the upper space 91 in FIG. 3.

Furthermore, the protective layer 84 includes the plurality of exposedspaces each having the longitudinal direction aligned with thelongitudinal direction of the first surface 102 a, i.e., the exposedspaces 95J1, 95J3 and 95J5, which are arranged side by side along theshort-length direction of the first surface 102 a. Therefore, stressgenerated, due to the difference in thermal expansion coefficientbetween the first protective layer 84 a and the element body 102 whenexposed to moisture, in the short-length direction of the first surface102 a and applied from the first protective layer 84 a to the elementbody 102 can be reduced as in the sensor elements 101F and 101G. As aresult, the element body 102 is less susceptible to cracking whenexposed to moisture, and the moisture resistance of the element body 102is further improved.

The exposed spaces 95J2, 95J4 and 95J6 facing the second surface 102 b,the exposed spaces 95J3 and 95J4 facing the third surface 102 c, and theexposed spaces 95J5 and 95J6 facing the fourth surface 102 d can alsoprovide similar advantageous effects with similar constitutions of theexposed spaces 95J1, 95J3 and 95J5 facing the first surface 102 a.

Twelfth Embodiment

FIG. 27 is a sectional view of a sensor element 101K according to atwelfth embodiment. FIG. 28 is a plan view of the sensor element 101Knear its front end. The sensor element 101K is similar to the sensorelement 101 according to the first embodiment except that the protectivelayer 84 includes a space 90K different from the space 90. The space 90Kincludes exposed spaces 95K1 to 95K4.

The exposed spaces 95K1 and 95K2 are the same spaces as the exposedspaces 95J1 and 95J2 in the sensor element 101J, respectively.

The exposed space 95K3 is a space arranged to extend over the upper,lower and left sides of the element body 102, and the first surface 102a, the second surface 102 b, and the third surface 102 c are exposed tothe exposed space 95K3. The exposed space 95K3 has a shape obtained bycutting away a part of an elliptic pillar. An inner surface of theexposed space 95K3 confronting the first surface 102 a (i.e., a downwardsurface thereof) is formed as a curved surface (corresponding to a partof an inner peripheral surface of a cylinder), which is obtained bybending a rectangle into a shape projecting toward the outside of thefirst protective layer 84 a (i.e., upward). Thus, the exposed space 95K3has a shape providing a space that gradually narrows toward a positionfarther away from the first surface 102 a (i.e., toward the upper side).An inner surface of the exposed space 95K3 confronting the secondsurface 102 b (i.e., an upward surface thereof) is formed as a curvedsurface (corresponding to a part of an inner peripheral surface of acylinder), which is obtained by bending a rectangle into a shapeprojecting toward the outside of the second protective layer 84 b (i.e.,downward). Thus, the exposed space 95K3 has a shape providing a spacethat gradually narrows toward a position farther away from the secondsurface 102 b (i.e., toward the lower side). Moreover, an inner surfaceof the exposed space 95K3 confronting the third surface 102 c (i.e., arightward surface thereof) is formed as a curved surface (correspondingto a part of an inner peripheral surface of a cylinder), which isobtained by bending a rectangle into a shape projecting toward theoutside of the third protective layer 84 c (i.e., leftward). Thus, theexposed space 95K3 has a shape providing a space that gradually narrowstoward a position farther away from the third surface 102 c (i.e.,toward the left side). A longitudinal direction of the exposed space95K3 is aligned with the longitudinal direction of each of the firstsurface 102 a, the second surface 102 b, and the third surface 102 c.

The exposed space 95K4 is a space arranged to extend over the upper,lower and right sides of the element body 102, and the first surface 102a, the second surface 102 b, and the fourth surface 102 d are exposed tothe exposed space 95K4. The exposed space 95K4 is symmetrical in shapeand layout to the exposed space 95K3 in the right-left direction.

The exposed spaces 95K1, 95K3 and 95K4 are arranged side by side alongthe short-length direction of the first surface 102 a. The exposedspaces 95K2, 95K3 and 95K4 are arranged side by side along theshort-length direction of the second surface 102 b.

Also in the sensor element 101K described above, as in the sensorelement 101, the moisture resistance of the element body 102 is furtherimproved with the presence of the exposed spaces 95K1 to 95K4. Moreover,each of the exposed spaces 95K1, 95K3 and 95K4 has the shape providingthe space that gradually narrows toward a position farther away from thefirst surface 102 a. In addition, the inner surfaces of the exposedspaces 95K1, 95K3 and 95K4 confronting the first surface 102 a areformed as the curved surfaces projecting outward. With those spaces eachhaving the above-mentioned shape, the reduction in the strength of thefirst protective layer 84 a can be suppressed in comparison with, forexample, a rectangular parallelepiped space having an inner surfaceparallel to the first surface 102 a, like the upper space 91 in FIG. 3.The exposed spaces 95K2, 95K3 and 95K4 facing the second surface 102 b,the exposed space 95K3 facing the third surface 102 c, and the exposedspace 95K4 facing the fourth surface 102 d can also provide similaradvantageous effects, i.e., the effects of suppressing the reduction inthe strength of the second to fourth protective layers 84 b to 84 d withsimilar constitutions of the exposed spaces 95K1, 95K3 and 95K4 facingthe first surface 102 a.

Furthermore, the protective layer 84 includes the plurality of exposedspaces each having the longitudinal direction aligned with thelongitudinal direction of the first surface 102 a, i.e., the exposedspaces 95K1, 95K3 and 95K4, which are arranged side by side along theshort-length direction of the first surface 102 a. Therefore, stressgenerated, due to the difference in thermal expansion coefficientbetween the first protective layer 84 a and the element body 102 whenexposed to moisture, in the short-length direction of the first surface102 a and applied from the first protective layer 84 a to the elementbody 102 can be reduced as in the sensor elements 101F and 101G. As aresult, the element body 102 is less susceptible to cracking whenexposed to moisture, and the moisture resistance of the element body 102is further improved. In addition, the protective layer 84 includes theplurality of exposed spaces each having the longitudinal directionaligned with the longitudinal direction of the second surface 102 b,i.e., the exposed spaces 95K2, 95K3 and 95K4, which are arranged side byside along the short-length direction of the second surface 102 b.Therefore, stress generated in the short-length direction of the secondsurface 102 b and applied from the second protective layer 84 b to theelement body 102 can be reduced as in the sensor elements 101F and 101G.As a result, the element body 102 is less susceptible to cracking whenexposed to moisture, and the moisture resistance of the element body 102is further improved.

The spaces in the protective layer 84 in each of the second to twelfthembodiments described above can also be formed, as in the firstembodiment, by employing a melt-disappearing material that disappearswith burning.

It is needless to say that the present invention is not limited to theabove-described embodiments, and that the present invention can be putinto practice in various forms insofar as falling within the technicalscope of the present invention.

While, in the first embodiment described above, the upper space 91 ispositioned in an overlapping relation to the center of a region of thefirst surface 102 a, the region being covered with the protective layer84, when viewed from the direction perpendicular to the first surface102 a, the present invention is not limited to that case. For example,the upper space 91 may be positioned, when viewed from the directionperpendicular to the first surface 102 a, in a relation overlapping themiddle of a region of the first surface 102 a in the back-forthdirection, the region being covered with the protective layer 84, butnot overlapping the middle of the relevant region in the right-leftdirection. As an alternative, the upper space 91 may be positioned in arelation overlapping the middle of the relevant region in the right-leftdirection, but not overlapping the middle of the relevant region in theback-forth direction, or it may be positioned in a relation overlappingneither the middle of the relevant region in the right-left directionnor the middle of the relevant region in the back-forth direction.

In the first embodiment described above, assuming that a region of thefirst surface 102 a where temperature is maximum in a state of theelement body 102 being heated by the heater 72 to the temperature (e.g.,about 800° C.) in an ordinary drive mode is called a maximum temperatureregion, the upper space 91 may be positioned in an overlapping relationto the maximum temperature region when viewed from the directionperpendicular to the first surface 102 a. With such an arrangement,since the region of the first surface 102 a where temperature ismaximized during the use of the sensor element 101 can be heat-insulatedby the upper space 91, the moisture resistance of the element body 102is further improved.

While, in the second embodiment described above, the spaces 91A to 94Aare provided with the communication holes H1 to H4 in a one-to-onerelation, respectively, the present invention is not limited to thatcase. One space may be provided with a plurality of communication holes.When the protective layer 84 includes a plurality of spaces, thosespaces may include one or more spaces opened to the outside and one ormore spaces not opened to the outside.

While, in the third embodiment described above, the exposed spaces 95B1and the non-exposed spaces 96B1 are positioned at different heights inthe up-down direction such that their positions do not overlap with eachother when viewed from the direction perpendicular to the up-downdirection, the present invention is not limited to that case, and thepositions of both the spaces may partly overlap with each other whenviewed from the direction perpendicular to the up-down direction. Whilethe exposed spaces 95B1 and the non-exposed spaces 96B1 are arranged atpositions deviated from each other in the back-forth direction, theirpositions in the back-forth direction may be the same. For example, thepositions of the exposed spaces 95B1 and the non-exposed spaces 96B1 maybe deviated only in the up-down direction, and may be the same in theback-forth direction and in the right-left direction. While the space90B includes inner and outer spaces arrayed in two stages (e.g., theexposed spaces 95B1 and the non-exposed spaces 96B1), the presentinvention is not limited to that case, and the space 90B may includespaces arrayed in three or more stages. The plural non-exposed spaces96B1 may be positioned at different heights in the up-down direction. Inthe third embodiment, the protective layer 84 is not always required toinclude the non-exposed spaces 96B1 to 96B4. Furthermore, in the thirdembodiment, the exposed spaces 95B1 and the non-exposed spaces 96B1 maybe arranged with a tendency that a density of the spaces graduallyincreases toward a position nearer to the middle of a region of thefirst surface 102 a, the region being covered with the protective layer84, when viewed from the direction perpendicular to the first surface102 a. With such an arrangement, heat insulation can be enhanced in thezone where temperature is comparatively more apt to become high, and themoisture resistance of the element body 102 is further improved.Likewise, the exposed spaces 95B1 and the non-exposed spaces 96B1 may bearranged with a tendency that a density of the spaces graduallyincreases toward a position nearer to the outer pump electrode 23 whenviewed from the direction perpendicular to the first surface 102 a. Theexpression “a tendency that a density of the spaces gradually increases”includes a tendency that the number of spaces per unit area graduallyincreases, and a tendency that the space size gradually increases. Theabove-described modifications are similarly applicable to the fourthembodiment as well.

While, in the fifth and sixth embodiments described above, the densityof the pillar portions P1 is changed by varying the number of the pillarportions P1 per unit area, the present invention is not limited to thatcase, and the density of the pillar portions P1 may be changed byvarying the thicknesses of the pillar portions P1. Furthermore, thepillar portions P1 may be arranged in a relation not overlapping theouter pump electrode 23 when viewed from the direction perpendicular tothe first surface 102 a. The density of the pillar portions P1 is notalways required to be specifically changed depending on locations.

While, in the fifth embodiment described above, the pillar portions P1are arranged with a tendency that a density of the pillar portions P1gradually increases from the middle of a region of the first surface 102a in the back-forth direction, the region being covered with theprotective layer 84, toward a position farther away from the middle whenviewed from the direction perpendicular to the first surface 102 a, thepresent invention is not limited to that case. The pillar portions P1may be arranged at a higher density at a position farther away from themiddle in the right-left direction, or at a higher density at a positionfarther away from the middle in the back-forth direction and the middlein the right-left direction. Likewise, the pillar portions P1 in thesixth embodiment may also be arranged at a density that is changed notonly in the back-forth direction, but also in the right-left direction.

While, in the seventh embodiment described above, one side of the firstexposed space 97F1 in the short-length direction of the first surface102 a is shorter than the other two sides, the present invention is notlimited to that case. It is just required that the longitudinaldirection of the first exposed space 97F1 is aligned with thelongitudinal direction of the first surface 102 a. Similarly, while, inthe seventh embodiment, one side of the second exposed space 98F1 in thelongitudinal direction of the first surface 102 a is shorter than theother two sides, the present invention is not limited to that case. Itis just required that the longitudinal direction of the second exposedspace 98F1 is aligned with the short-length direction of the firstsurface 102 a.

While, in the eleventh embodiment described above, the exposed space95J3 is arranged to extend over the upper side and the left side of theelement body 102, the present invention is not limited to that case. Forexample, the protective layer 84 may include, instead of the exposedspace 95J3, a space having a semi-elliptic pillar shape and formed onthe upper side of the element body 102, and a space having asemi-elliptic pillar shape and formed on the left side of the elementbody 102. The above point is similarly applied to the exposed spaces95J4 to 95J6 and to the exposed spaces 95K3 and 95K4 in the twelfthembodiment.

While, in the tenth to twelfth embodiments described above, thelongitudinal direction of each of the spaces included in the protectivelayer 84 is aligned with the back-forth direction, the present inventionis not limited to that case. The effect of suppressing the reduction inthe strength of the first protective layer 84 a can be obtained insofaras the exposed space to which the first surface 102 a is exposed has ashape providing a space that gradually narrows toward a position fartheraway from the first surface 102 a. For example, the upper space 91I inFIG. 23 may have a triangular pyramid shape. The exposed space 95J1 inFIG. 25 may have a semi-spherical shape.

While, in the first to twelfth embodiments described above, the space(s)positioned on the upper side of the element body 102 and the space(s)positioned on the lower side thereof are symmetrical in the up-downdirection and the space(s) positioned on the left side of the elementbody 102 and the space(s) positioned on the right side thereof aresymmetrical in the right-left direction, the present invention is notlimited to that case. Furthermore, while, in the first to twelfthembodiments described above, each of the first to fourth protectivelayers 84 a to 84 d includes the space(s), the present invention is notlimited to that case. The protective layer 84 is just required toinclude one or more exposed spaces to which the first surface 102 a isexposed. For example, in the first embodiment, the protective layer 84is just required to include the upper space 91, and one or more of thelower space 92, the left space 93, and the right space 94 may beomitted. The fifth protective layer 84 e may include one or more spacessimilarly to the first to fourth protective layers 84 a to 84 d.

While, in the first to twelfth embodiments described above, theprotective layer 84 includes the first to fifth protective layers 84 ato 84 e, the present invention is not limited to that case. Theprotective layer 84 is just required to include at least the firstprotective layer 84 a. Moreover, the first protective layer 84 a is justrequired to cover at least a part of the first surface 102 a.

While, in the above embodiments, sizes of the spaces included in theprotective layer 84 have not been described in particular, the spacesare just required to have sizes allowing the spaces to bedistinguishable from pores in the protective layer 84. For example, thevolume of one space may be 12500 μm³ or more. Assuming a volume rate ofthe spaces to be denoted by (volume rate)=(total volume of the spaces inthe protective layer 84)/(volume of the protective layer 84)×100, thevolume rate may be 60% or less. The “volume of the protective layer 84”represents a value including the volume of the spaces in the protectivelayer 84.

Though not described in the above first to twelfth embodiments, thetotal volume of one or more spaces present above the first surface 102 ais preferably 0.03 mm³ or more. On that condition, the effect ofimproving the moisture resistance of the element body with the provisionof the one or more spaces can be obtained reliably. For example, in thefirst to tenth embodiments described above, the total volume for each ofthe upper spaces 91 and 91A to 91I is preferably 0.03 mm³ or more. Inthe eleventh embodiment, the total volume of the exposed space 95J1, aportion of the exposed space 95J3, the portion being positioned on theupper side of the first surface 102 a, and a portion of the exposedspace 95J5, the portion being positioned on the upper side of the firstsurface 102 a, is preferably 0.03 mm³ or more. In the twelfthembodiment, the total volume of the exposed space 95K1, a portion of theexposed space 95K3, the portion being positioned on the upper side ofthe first surface 102 a, and a portion of the exposed space 95K4, theportion being positioned on the upper side of the first surface 102 a,is preferably 0.03 mm³ or more. Similarly, the total volume of one ormore spaces present on the lower side of the second surface 102 b ispreferably 0.03 mm³ or more. The total volume of one or more spacespresent on the left side of the third surface 102 c is preferably 0.015mm³ or more. The total volume of one or more spaces present on the rightside of the fourth surface 102 d is preferably 0.015 mm³ or more. Theexpression “the upper side of the first surface 102 a” implies a regioncontaining, e.g., the upper left side and the upper right side of thefirst surface 102 a without being limited to the side just above thefirst surface 102 a. Similarly, the expression “the lower side of thesecond surface 102 b” is not limited to the side just under the secondsurface 102 b. The above point is similarly applied to “the left side ofthe third surface 102 c” and “the right side of the fourth surface 102d”.

Though not described in the above first to twelfth embodiments, whenthere is one or more spaces extending over two adjacent surfaces (i.e.,surfaces having one common side), the total volume of the one or morespaces is preferably 0.002 mm³ or more. With such a feature, the effectof improving the moisture resistance of the element body with theprovision of the one or more spaces can be obtained more reliably. Forexample, in the eleventh embodiment, the volume of the exposed space95J3 extending over the first surface 102 a and the third surface 102 cis preferably 0.002 mm³ or more. Similarly, the volume of the exposedspace 95J4 extending over the second surface 102 b and the third surface102 c, the volume of the exposed space 95J5 extending over the firstsurface 102 a and the fourth surface 102 d, and the volume of theexposed space 95J6 extending over the second surface 102 b and thefourth surface 102 d are each also preferably 0.002 mm³ or more. Theexpression “the space extending over two adjacent surfaces” implies aspace that is present in each of the directions perpendicular to thosetwo surfaces. For example, the exposed space 95J3 is present in the(upward) direction perpendicular to the first surface 102 a and in the(leftward) direction perpendicular to the third surface 102 c, and it isa space extending over the first surface 102 a and the third surface 102c.

The protective layer 84 may include an appropriate combination of two ormore among not only the spaces constituting the spaces 90 and 90A to 90Kin the first to twelfth embodiments described above, but also spacesmodified from those spaces. That combination contains the case where theprotective layer 84 includes different types of the spaces, and the casewhere the protective layer 84 includes spaces having the above-describedfeatures regarding the different types of shapes and layouts of thespaces.

For example, one or more of the above-described spaces except for thespaces in the second embodiment may be provided with an opening that iscommunicated with the outside of the protective layer 84. When the spaceis provided with the opening, a communication hole for communicating thespace and the outside with each other like may be provided like thecommunication holes H1 to H4 in the second embodiment, or the space maybe extended to the surface of the protective layer 84 to be directlyopened to the outside. The opening associated with the space ispreferably formed in such an appropriate size as being able not only tosuppress moisture from directly entering the inside, but also to releaseheat within the space through the opening. An opening area may be set to100 μm² to 1000 μm², for example.

As still another example, regarding the spaces in the embodiments otherthan the first embodiment, at least one of the exposed spaces may bepositioned in an overlapping relation to the middle of a region of thefirst surface 102 a, the region being covered with the protective layer84, or at least a part of the exposed space may be positioned in anoverlapping relation to the outer pump electrode 23 when viewed from thedirection perpendicular to the first surface 102 a.

The present application claims priority from Japanese Patent ApplicationNo. 2015-066700 filed on Mar. 27, 2015, and Japanese Patent ApplicationNo. 2016-049722 filed on Mar. 14, 2016, the entire contents of which areincorporated herein by reference.

What is claimed is:
 1. A sensor element including: an element bodyhaving an elongate rectangular parallelepiped shape and including solidelectrolyte layers with oxygen ion conductivity, an outer electrodedisposed on a first surface that is one of surfaces of the element body,and a porous protective layer covering at least a part of the firstsurface of the element body and including one or more exposed spaces towhich the first surface is exposed, the one or more exposed spaces beingprovided within the porous protective layer such that a portion of theporous protective layer forms a first side surface and a second sidesurface of each of the one or more exposed spaces, the first sidesurface and the second side surface facing each other and being locatedon opposite sides of each of the one or more exposed spaces, the elementbody having a heater embedded therein, the porous protective layercovering at least a part of a second surface of the element body, thesecond surface being opposite the first surface of the element body, andthe one or more exposed spaces being provided in addition to pores ofthe porous protective layer and each having a volume of 12500 μm³ ormore.
 2. The sensor element according to claim 1, wherein at least oneof the one or more exposed spaces is positioned in an overlappingrelation to a middle of a region of the first surface, the region beingcovered with the porous protective layer, when viewed from a directionperpendicular to the first surface.
 3. The sensor element according toclaim 1, wherein at least one of the one or more exposed spaces ispositioned such that at least a part of the at least one exposed spaceoverlaps at least a part of the outer electrode when viewed from adirection perpendicular to the first surface.
 4. The sensor elementaccording to claim 1, wherein at least one of the one or more exposedspaces is provided with an opening in communication with an outside ofthe porous protective layer.
 5. The sensor element according to claim 1,wherein the porous protective layer includes the one or more exposedspaces, and a plurality of non-exposed spaces to which the first surfaceis not exposed, and which are present at positions deviated from atleast one of the one or more exposed spaces in a direction perpendicularto the first surface.
 6. The sensor element according to claim 1,wherein the porous protective layer includes, for at least one of theone or more exposed spaces, one or more pillar portions that hold the atleast one exposed space in a direction perpendicular to the firstsurface.
 7. The sensor element according to claim 6, wherein the one ormore pillar portions includes a plurality of pillar portions, and theplurality of pillar portions are arranged with a tendency that a densityof the plurality of pillar portions gradually increases from a middle ofa region of the first surface, the region being covered with the porousprotective layer, toward a position farther away from the middle whenviewed from the direction perpendicular to the first surface.
 8. Thesensor element according to claim 6, wherein the one or more pillarportions includes a plurality of pillar portions, and the plurality ofpillar portions are arranged with a tendency that a density of theplurality of pillar portions gradually increases toward a positionfarther away from the outer electrode when viewed from the directionperpendicular to the first surface.
 9. The sensor element according toclaim 6, wherein the one or more pillar portions includes a plurality ofpillar portions, and the plurality of pillar portions are arranged witha tendency that a density of the plurality of pillar portions graduallyincreases toward a position nearer to a middle of a region of the firstsurface, the region being covered with the porous protective layer, whenviewed from the direction perpendicular to the first surface.
 10. Thesensor element according to claim 1, wherein the one or more exposedspaces includes a plurality of exposed spaces each having a longitudinaldirection aligned with a longitudinal direction of the first surface anddisposed side by side along a short-length direction of the firstsurface.
 11. The sensor element according to claim 1, wherein the one ormore exposed spaces includes a plurality of exposed spaces each having alongitudinal direction aligned with a short-length direction of thefirst surface and disposed side by side along a longitudinal directionof the first surface.
 12. The sensor element according to claim 1,wherein the porous protective layer includes a plurality of firstexposed spaces of the one or more exposed spaces that each have alongitudinal direction aligned with a longitudinal direction of thefirst surface, and that are disposed side by side along a short-lengthdirection of the first surface, and a plurality of second exposed spacesof the one or more exposed spaces that each have a longitudinaldirection aligned with the short-length direction of the first surfaceand intersecting the plurality of first exposed spaces, and that aredisposed side by side along the longitudinal direction of the firstsurface.
 13. The sensor element according to claim 1, wherein at leastone of the one or more exposed spaces has a shape with a tendency thatthe at least one exposed space gradually narrows toward a positionfarther away from the first surface.
 14. The sensor element according toclaim 1, wherein at least one of the one or more exposed spaces has atleast two inner surfaces inclined in such directions that the at leasttwo inner surfaces come closer to each other toward a position fartheraway from the first surface.
 15. The sensor element according to claim1, wherein at least one of the one or more exposed spaces has an innersurface opposing the first surface and formed as a curved surfaceprojecting outward.
 16. A gas sensor including the sensor elementaccording to claim 1.